cpython-lru_cache-weakref: Doc/tutorial/classes.rst annotate

annotate Doc/tutorial/classes.rst @ 47787:b32101c19bf8 legacy-trunk

Grammar fixes

author	Andrew M. Kuchling <amk@amk.ca>
date	Fri, 20 Jun 2008 23:14:32 +0000
parents	a4c265565cb6
children	3f6d43d5755c

rev	line source
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-classes:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 *******
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 Classes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 *******
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7 Python's class mechanism adds classes to the language with a minimum of new
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 syntax and semantics. It is a mixture of the class mechanisms found in C++ and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9 Modula-3. As is true for modules, classes in Python do not put an absolute
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10 barrier between definition and user, but rather rely on the politeness of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11 user not to "break into the definition." The most important features of classes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12 are retained with full power, however: the class inheritance mechanism allows
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13 multiple base classes, a derived class can override any methods of its base
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14 class or classes, and a method can call the method of a base class with the same
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 name. Objects can contain an arbitrary amount of private data.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17 In C++ terminology, all class members (including the data members) are public,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18 and all member functions are virtual. There are no special constructors or
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19 destructors. As in Modula-3, there are no shorthands for referencing the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	20 object's members from its methods: the method function is declared with an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21 explicit first argument representing the object, which is provided implicitly by
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 the call. As in Smalltalk, classes themselves are objects, albeit in the wider
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23 sense of the word: in Python, all data types are objects. This provides
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24 semantics for importing and renaming. Unlike C++ and Modula-3, built-in types
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 can be used as base classes for extension by the user. Also, like in C++ but
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 unlike in Modula-3, most built-in operators with special syntax (arithmetic
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27 operators, subscripting etc.) can be redefined for class instances.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30 .. _tut-terminology:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32 A Word About Terminology
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	33 ========================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35 Lacking universally accepted terminology to talk about classes, I will make
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 occasional use of Smalltalk and C++ terms. (I would use Modula-3 terms, since
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 its object-oriented semantics are closer to those of Python than C++, but I
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38 expect that few readers have heard of it.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40 Objects have individuality, and multiple names (in multiple scopes) can be bound
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41 to the same object. This is known as aliasing in other languages. This is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42 usually not appreciated on a first glance at Python, and can be safely ignored
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 when dealing with immutable basic types (numbers, strings, tuples). However,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 aliasing has an (intended!) effect on the semantics of Python code involving
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45 mutable objects such as lists, dictionaries, and most types representing
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46 entities outside the program (files, windows, etc.). This is usually used to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 the benefit of the program, since aliases behave like pointers in some respects.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48 For example, passing an object is cheap since only a pointer is passed by the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49 implementation; and if a function modifies an object passed as an argument, the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 caller will see the change --- this eliminates the need for two different
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 argument passing mechanisms as in Pascal.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 .. _tut-scopes:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56 Python Scopes and Name Spaces
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57 =============================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	58
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	59 Before introducing classes, I first have to tell you something about Python's
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60 scope rules. Class definitions play some neat tricks with namespaces, and you
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 need to know how scopes and namespaces work to fully understand what's going on.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62 Incidentally, knowledge about this subject is useful for any advanced Python
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63 programmer.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	65 Let's begin with some definitions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 A namespace is a mapping from names to objects. Most namespaces are currently
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68 implemented as Python dictionaries, but that's normally not noticeable in any
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	69 way (except for performance), and it may change in the future. Examples of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 namespaces are: the set of built-in names (functions such as :func:`abs`, and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	71 built-in exception names); the global names in a module; and the local names in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72 a function invocation. In a sense the set of attributes of an object also form
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73 a namespace. The important thing to know about namespaces is that there is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74 absolutely no relation between names in different namespaces; for instance, two
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75 different modules may both define a function "maximize" without confusion ---
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 users of the modules must prefix it with the module name.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78 By the way, I use the word attribute for any name following a dot --- for
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 example, in the expression ``z.real``, ``real`` is an attribute of the object
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 ``z``. Strictly speaking, references to names in modules are attribute
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81 references: in the expression ``modname.funcname``, ``modname`` is a module
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 object and ``funcname`` is an attribute of it. In this case there happens to be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83 a straightforward mapping between the module's attributes and the global names
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 defined in the module: they share the same namespace! [#]_
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 Attributes may be read-only or writable. In the latter case, assignment to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 attributes is possible. Module attributes are writable: you can write
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 ``modname.the_answer = 42``. Writable attributes may also be deleted with the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 :keyword:`del` statement. For example, ``del modname.the_answer`` will remove
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 the attribute :attr:`the_answer` from the object named by ``modname``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 Name spaces are created at different moments and have different lifetimes. The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 namespace containing the built-in names is created when the Python interpreter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 starts up, and is never deleted. The global namespace for a module is created
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95 when the module definition is read in; normally, module namespaces also last
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 until the interpreter quits. The statements executed by the top-level
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97 invocation of the interpreter, either read from a script file or interactively,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 are considered part of a module called :mod:`__main__`, so they have their own
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 global namespace. (The built-in names actually also live in a module; this is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 called :mod:`__builtin__`.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102 The local namespace for a function is created when the function is called, and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 deleted when the function returns or raises an exception that is not handled
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104 within the function. (Actually, forgetting would be a better way to describe
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 what actually happens.) Of course, recursive invocations each have their own
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 local namespace.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108 A scope is a textual region of a Python program where a namespace is directly
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109 accessible. "Directly accessible" here means that an unqualified reference to a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110 name attempts to find the name in the namespace.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 Although scopes are determined statically, they are used dynamically. At any
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 time during execution, there are at least three nested scopes whose namespaces
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 are directly accessible: the innermost scope, which is searched first, contains
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115 the local names; the namespaces of any enclosing functions, which are searched
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 starting with the nearest enclosing scope; the middle scope, searched next,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 contains the current module's global names; and the outermost scope (searched
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 last) is the namespace containing built-in names.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 If a name is declared global, then all references and assignments go directly to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 the middle scope containing the module's global names. Otherwise, all variables
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 found outside of the innermost scope are read-only (an attempt to write to such
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 a variable will simply create a new local variable in the innermost scope,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 leaving the identically named outer variable unchanged).
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125
45687 15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	126 .. XXX mention nonlocal
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	127
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 Usually, the local scope references the local names of the (textually) current
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 function. Outside functions, the local scope references the same namespace as
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 the global scope: the module's namespace. Class definitions place yet another
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 namespace in the local scope.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 It is important to realize that scopes are determined textually: the global
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134 scope of a function defined in a module is that module's namespace, no matter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135 from where or by what alias the function is called. On the other hand, the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 actual search for names is done dynamically, at run time --- however, the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137 language definition is evolving towards static name resolution, at "compile"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138 time, so don't rely on dynamic name resolution! (In fact, local variables are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139 already determined statically.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140
45687 15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	141 A special quirk of Python is that -- if no :keyword:`global` or
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	142 :keyword:`nonlocal` statement is in effect -- assignments to names always go
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	143 into the innermost scope. Assignments do not copy data --- they just bind names
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	144 to objects. The same is true for deletions: the statement ``del x`` removes the
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	145 binding of ``x`` from the namespace referenced by the local scope. In fact, all
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	146 operations that introduce new names use the local scope: in particular, import
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	147 statements and function definitions bind the module or function name in the
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	148 local scope. (The :keyword:`global` statement can be used to indicate that
15746bf4a0a6 Little clarification of assignments. Georg Brandl <georg@python.org> parents: 44383 diff changeset	149 particular variables live in the global scope.)
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 .. _tut-firstclasses:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 A First Look at Classes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155 =======================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 Classes introduce a little bit of new syntax, three new object types, and some
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 new semantics.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 .. _tut-classdefinition:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	162
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 Class Definition Syntax
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 -----------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166 The simplest form of class definition looks like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 class ClassName:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 <statement-1>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	171 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	172 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	173 <statement-N>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	174
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175 Class definitions, like function definitions (:keyword:`def` statements) must be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176 executed before they have any effect. (You could conceivably place a class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 definition in a branch of an :keyword:`if` statement, or inside a function.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	178
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	179 In practice, the statements inside a class definition will usually be function
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180 definitions, but other statements are allowed, and sometimes useful --- we'll
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181 come back to this later. The function definitions inside a class normally have
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 a peculiar form of argument list, dictated by the calling conventions for
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	183 methods --- again, this is explained later.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	184
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 When a class definition is entered, a new namespace is created, and used as the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 local scope --- thus, all assignments to local variables go into this new
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 namespace. In particular, function definitions bind the name of the new
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	188 function here.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	189
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190 When a class definition is left normally (via the end), a class object is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 created. This is basically a wrapper around the contents of the namespace
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192 created by the class definition; we'll learn more about class objects in the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 next section. The original local scope (the one in effect just before the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194 definition was entered) is reinstated, and the class object is bound here to the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195 class name given in the class definition header (:class:`ClassName` in the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196 example).
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 .. _tut-classobjects:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 Class Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	202 -------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 Class objects support two kinds of operations: attribute references and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	205 instantiation.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	207 Attribute references use the standard syntax used for all attribute references
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 in Python: ``obj.name``. Valid attribute names are all the names that were in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 the class's namespace when the class object was created. So, if the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 definition looked like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212 class MyClass:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213 "A simple example class"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 i = 12345
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215 def f(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216 return 'hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 then ``MyClass.i`` and ``MyClass.f`` are valid attribute references, returning
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219 an integer and a function object, respectively. Class attributes can also be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 assigned to, so you can change the value of ``MyClass.i`` by assignment.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 :attr:`__doc__` is also a valid attribute, returning the docstring belonging to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222 the class: ``"A simple example class"``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	223
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224 Class instantiation uses function notation. Just pretend that the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 object is a parameterless function that returns a new instance of the class.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226 For example (assuming the above class)::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228 x = MyClass()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	229
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 creates a new instance of the class and assigns this object to the local
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231 variable ``x``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	232
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	233 The instantiation operation ("calling" a class object) creates an empty object.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234 Many classes like to create objects with instances customized to a specific
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	235 initial state. Therefore a class may define a special method named
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 :meth:`__init__`, like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	237
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	238 def __init__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	239 self.data = []
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	240
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	241 When a class defines an :meth:`__init__` method, class instantiation
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	242 automatically invokes :meth:`__init__` for the newly-created class instance. So
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243 in this example, a new, initialized instance can be obtained by::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	244
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	245 x = MyClass()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	246
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	247 Of course, the :meth:`__init__` method may have arguments for greater
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	248 flexibility. In that case, arguments given to the class instantiation operator
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	249 are passed on to :meth:`__init__`. For example, ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	250
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	251 >>> class Complex:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	252 ... def __init__(self, realpart, imagpart):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	253 ... self.r = realpart
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	254 ... self.i = imagpart
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	255 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	256 >>> x = Complex(3.0, -4.5)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	257 >>> x.r, x.i
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	258 (3.0, -4.5)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	259
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	260
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	261 .. _tut-instanceobjects:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	262
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	263 Instance Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	264 ----------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	265
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266 Now what can we do with instance objects? The only operations understood by
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	267 instance objects are attribute references. There are two kinds of valid
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	268 attribute names, data attributes and methods.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	269
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	270 data attributes correspond to "instance variables" in Smalltalk, and to "data
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	271 members" in C++. Data attributes need not be declared; like local variables,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	272 they spring into existence when they are first assigned to. For example, if
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 ``x`` is the instance of :class:`MyClass` created above, the following piece of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 code will print the value ``16``, without leaving a trace::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 x.counter = 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 while x.counter < 10:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 x.counter = x.counter * 2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 print x.counter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	280 del x.counter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	281
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	282 The other kind of instance attribute reference is a method. A method is a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283 function that "belongs to" an object. (In Python, the term method is not unique
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	284 to class instances: other object types can have methods as well. For example,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	285 list objects have methods called append, insert, remove, sort, and so on.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 However, in the following discussion, we'll use the term method exclusively to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287 mean methods of class instance objects, unless explicitly stated otherwise.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289 .. index:: object: method
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 Valid method names of an instance object depend on its class. By definition,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292 all attributes of a class that are function objects define corresponding
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293 methods of its instances. So in our example, ``x.f`` is a valid method
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294 reference, since ``MyClass.f`` is a function, but ``x.i`` is not, since
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 ``MyClass.i`` is not. But ``x.f`` is not the same thing as ``MyClass.f`` --- it
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296 is a method object, not a function object.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 .. _tut-methodobjects:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	301 Method Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302 --------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304 Usually, a method is called right after it is bound::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	305
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306 x.f()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 In the :class:`MyClass` example, this will return the string ``'hello world'``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	309 However, it is not necessary to call a method right away: ``x.f`` is a method
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310 object, and can be stored away and called at a later time. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312 xf = x.f
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313 while True:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	314 print xf()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316 will continue to print ``hello world`` until the end of time.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	317
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 What exactly happens when a method is called? You may have noticed that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 ``x.f()`` was called without an argument above, even though the function
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320 definition for :meth:`f` specified an argument. What happened to the argument?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321 Surely Python raises an exception when a function that requires an argument is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322 called without any --- even if the argument isn't actually used...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 Actually, you may have guessed the answer: the special thing about methods is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325 that the object is passed as the first argument of the function. In our
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326 example, the call ``x.f()`` is exactly equivalent to ``MyClass.f(x)``. In
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 general, calling a method with a list of n arguments is equivalent to calling
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328 the corresponding function with an argument list that is created by inserting
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329 the method's object before the first argument.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	330
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331 If you still don't understand how methods work, a look at the implementation can
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	332 perhaps clarify matters. When an instance attribute is referenced that isn't a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	333 data attribute, its class is searched. If the name denotes a valid class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	334 attribute that is a function object, a method object is created by packing
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	335 (pointers to) the instance object and the function object just found together in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	336 an abstract object: this is the method object. When the method object is called
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	337 with an argument list, it is unpacked again, a new argument list is constructed
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	338 from the instance object and the original argument list, and the function object
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	339 is called with this new argument list.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	340
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	341
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	342 .. _tut-remarks:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	343
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	344 Random Remarks
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	345 ==============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	346
44383 3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	347 .. These should perhaps be placed more carefully...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	348
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	349 Data attributes override method attributes with the same name; to avoid
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	350 accidental name conflicts, which may cause hard-to-find bugs in large programs,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	351 it is wise to use some kind of convention that minimizes the chance of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	352 conflicts. Possible conventions include capitalizing method names, prefixing
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	353 data attribute names with a small unique string (perhaps just an underscore), or
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	354 using verbs for methods and nouns for data attributes.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	355
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	356 Data attributes may be referenced by methods as well as by ordinary users
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	357 ("clients") of an object. In other words, classes are not usable to implement
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	358 pure abstract data types. In fact, nothing in Python makes it possible to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	359 enforce data hiding --- it is all based upon convention. (On the other hand,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360 the Python implementation, written in C, can completely hide implementation
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361 details and control access to an object if necessary; this can be used by
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	362 extensions to Python written in C.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	363
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	364 Clients should use data attributes with care --- clients may mess up invariants
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 maintained by the methods by stamping on their data attributes. Note that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	366 clients may add data attributes of their own to an instance object without
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367 affecting the validity of the methods, as long as name conflicts are avoided ---
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	368 again, a naming convention can save a lot of headaches here.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370 There is no shorthand for referencing data attributes (or other methods!) from
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371 within methods. I find that this actually increases the readability of methods:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 there is no chance of confusing local variables and instance variables when
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373 glancing through a method.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375 Often, the first argument of a method is called ``self``. This is nothing more
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	376 than a convention: the name ``self`` has absolutely no special meaning to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	377 Python. (Note, however, that by not following the convention your code may be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378 less readable to other Python programmers, and it is also conceivable that a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379 class browser program might be written that relies upon such a convention.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	381 Any function object that is a class attribute defines a method for instances of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	382 that class. It is not necessary that the function definition is textually
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383 enclosed in the class definition: assigning a function object to a local
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384 variable in the class is also ok. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 # Function defined outside the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 def f1(self, x, y):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 return min(x, x+y)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390 class C:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 f = f1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392 def g(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 return 'hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394 h = g
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 Now ``f``, ``g`` and ``h`` are all attributes of class :class:`C` that refer to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 function objects, and consequently they are all methods of instances of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 :class:`C` --- ``h`` being exactly equivalent to ``g``. Note that this practice
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 usually only serves to confuse the reader of a program.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 Methods may call other methods by using method attributes of the ``self``
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402 argument::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 class Bag:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405 def __init__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 self.data = []
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	407 def add(self, x):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408 self.data.append(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409 def addtwice(self, x):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410 self.add(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411 self.add(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	412
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	413 Methods may reference global names in the same way as ordinary functions. The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414 global scope associated with a method is the module containing the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	415 definition. (The class itself is never used as a global scope!) While one
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	416 rarely encounters a good reason for using global data in a method, there are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	417 many legitimate uses of the global scope: for one thing, functions and modules
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	418 imported into the global scope can be used by methods, as well as functions and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	419 classes defined in it. Usually, the class containing the method is itself
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420 defined in this global scope, and in the next section we'll find some good
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 reasons why a method would want to reference its own class!
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	422
45688 a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	423 Each value is an object, and therefore has a class (also called its type).
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	424 It is stored as ``object.__class__``.
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	425
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	427 .. _tut-inheritance:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	428
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	429 Inheritance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 ===========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432 Of course, a language feature would not be worthy of the name "class" without
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	433 supporting inheritance. The syntax for a derived class definition looks like
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	434 this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	435
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436 class DerivedClassName(BaseClassName):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 <statement-1>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	439 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	440 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441 <statement-N>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443 The name :class:`BaseClassName` must be defined in a scope containing the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	444 derived class definition. In place of a base class name, other arbitrary
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	445 expressions are also allowed. This can be useful, for example, when the base
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	446 class is defined in another module::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	447
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	448 class DerivedClassName(modname.BaseClassName):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	449
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	450 Execution of a derived class definition proceeds the same as for a base class.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451 When the class object is constructed, the base class is remembered. This is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452 used for resolving attribute references: if a requested attribute is not found
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453 in the class, the search proceeds to look in the base class. This rule is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454 applied recursively if the base class itself is derived from some other class.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 There's nothing special about instantiation of derived classes:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457 ``DerivedClassName()`` creates a new instance of the class. Method references
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 are resolved as follows: the corresponding class attribute is searched,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459 descending down the chain of base classes if necessary, and the method reference
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 is valid if this yields a function object.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 Derived classes may override methods of their base classes. Because methods
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 have no special privileges when calling other methods of the same object, a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	464 method of a base class that calls another method defined in the same base class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465 may end up calling a method of a derived class that overrides it. (For C++
44383 3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	466 programmers: all methods in Python are effectively ``virtual``.)
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468 An overriding method in a derived class may in fact want to extend rather than
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469 simply replace the base class method of the same name. There is a simple way to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470 call the base class method directly: just call ``BaseClassName.methodname(self,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471 arguments)``. This is occasionally useful to clients as well. (Note that this
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472 only works if the base class is defined or imported directly in the global
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473 scope.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474
45688 a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	475 Python has two builtin functions that work with inheritance:
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	476
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	477 * Use :func:`isinstance` to check an object's type: ``isinstance(obj, int)``
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	478 will be ``True`` only if ``obj.__class__`` is :class:`int` or some class
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	479 derived from :class:`int`.
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	480
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	481 * Use :func:`issubclass` to check class inheritance: ``issubclass(bool, int)``
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	482 is ``True`` since :class:`bool` is a subclass of :class:`int`. However,
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	483 ``issubclass(unicode, str)`` is ``False`` since :class:`unicode` is not a
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	484 subclass of :class:`str` (they only share a common ancestor,
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	485 :class:`basestring`).
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	486
a4c265565cb6 Add isinstance/issubclass to tutorial. Georg Brandl <georg@python.org> parents: 45687 diff changeset	487
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489 .. _tut-multiple:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	490
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	491 Multiple Inheritance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	492 --------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 Python supports a limited form of multiple inheritance as well. A class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 definition with multiple base classes looks like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 class DerivedClassName(Base1, Base2, Base3):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	498 <statement-1>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 <statement-N>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	504 For old-style classes, the only rule is depth-first, left-to-right. Thus, if an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	505 attribute is not found in :class:`DerivedClassName`, it is searched in
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506 :class:`Base1`, then (recursively) in the base classes of :class:`Base1`, and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 only if it is not found there, it is searched in :class:`Base2`, and so on.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509 (To some people breadth first --- searching :class:`Base2` and :class:`Base3`
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 before the base classes of :class:`Base1` --- looks more natural. However, this
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511 would require you to know whether a particular attribute of :class:`Base1` is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 actually defined in :class:`Base1` or in one of its base classes before you can
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	513 figure out the consequences of a name conflict with an attribute of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514 :class:`Base2`. The depth-first rule makes no differences between direct and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515 inherited attributes of :class:`Base1`.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516
43631 03056365a1b6 Add :term:s for "new-style class". Georg Brandl <georg@python.org> parents: 43629 diff changeset	517 For :term:`new-style class`\es, the method resolution order changes dynamically
03056365a1b6 Add :term:s for "new-style class". Georg Brandl <georg@python.org> parents: 43629 diff changeset	518 to support cooperative calls to :func:`super`. This approach is known in some
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519 other multiple-inheritance languages as call-next-method and is more powerful
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 than the super call found in single-inheritance languages.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 With new-style classes, dynamic ordering is necessary because all cases of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523 multiple inheritance exhibit one or more diamond relationships (where one at
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	524 least one of the parent classes can be accessed through multiple paths from the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	525 bottommost class). For example, all new-style classes inherit from
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526 :class:`object`, so any case of multiple inheritance provides more than one path
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527 to reach :class:`object`. To keep the base classes from being accessed more
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	528 than once, the dynamic algorithm linearizes the search order in a way that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529 preserves the left-to-right ordering specified in each class, that calls each
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530 parent only once, and that is monotonic (meaning that a class can be subclassed
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 without affecting the precedence order of its parents). Taken together, these
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532 properties make it possible to design reliable and extensible classes with
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 multiple inheritance. For more detail, see
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 http://www.python.org/download/releases/2.3/mro/.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537 .. _tut-private:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 Private Variables
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540 =================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	541
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 There is limited support for class-private identifiers. Any identifier of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	543 form ``__spam`` (at least two leading underscores, at most one trailing
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 underscore) is textually replaced with ``_classname__spam``, where ``classname``
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 is the current class name with leading underscore(s) stripped. This mangling is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 done without regard to the syntactic position of the identifier, so it can be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	547 used to define class-private instance and class variables, methods, variables
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548 stored in globals, and even variables stored in instances. private to this class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549 on instances of other classes. Truncation may occur when the mangled name
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550 would be longer than 255 characters. Outside classes, or when the class name
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 consists of only underscores, no mangling occurs.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553 Name mangling is intended to give classes an easy way to define "private"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 instance variables and methods, without having to worry about instance variables
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555 defined by derived classes, or mucking with instance variables by code outside
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 the class. Note that the mangling rules are designed mostly to avoid accidents;
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 it still is possible for a determined soul to access or modify a variable that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558 is considered private. This can even be useful in special circumstances, such
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 as in the debugger, and that's one reason why this loophole is not closed.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 (Buglet: derivation of a class with the same name as the base class makes use of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 private variables of the base class possible.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 Notice that code passed to ``exec``, ``eval()`` or ``execfile()`` does not
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 consider the classname of the invoking class to be the current class; this is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 similar to the effect of the ``global`` statement, the effect of which is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566 likewise restricted to code that is byte-compiled together. The same
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	567 restriction applies to ``getattr()``, ``setattr()`` and ``delattr()``, as well
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 as when referencing ``__dict__`` directly.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	569
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	570
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	571 .. _tut-odds:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	572
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 Odds and Ends
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574 =============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576 Sometimes it is useful to have a data type similar to the Pascal "record" or C
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577 "struct", bundling together a few named data items. An empty class definition
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578 will do nicely::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	579
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	580 class Employee:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	581 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 john = Employee() # Create an empty employee record
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 # Fill the fields of the record
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586 john.name = 'John Doe'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 john.dept = 'computer lab'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 john.salary = 1000
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 A piece of Python code that expects a particular abstract data type can often be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591 passed a class that emulates the methods of that data type instead. For
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592 instance, if you have a function that formats some data from a file object, you
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	593 can define a class with methods :meth:`read` and :meth:`readline` that get the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594 data from a string buffer instead, and pass it as an argument.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595
44383 3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	596 .. (Unfortunately, this technique has its limitations: a class can't define
3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	597 operations that are accessed by special syntax such as sequence subscripting
3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	598 or arithmetic operators, and assigning such a "pseudo-file" to sys.stdin will
3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	599 not cause the interpreter to read further input from it.)
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	600
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	601 Instance method objects have attributes, too: ``m.im_self`` is the instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	602 object with the method :meth:`m`, and ``m.im_func`` is the function object
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	603 corresponding to the method.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	604
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	605
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	606 .. _tut-exceptionclasses:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	608 Exceptions Are Classes Too
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609 ==========================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611 User-defined exceptions are identified by classes as well. Using this mechanism
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 it is possible to create extensible hierarchies of exceptions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 There are two new valid (semantic) forms for the raise statement::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 raise Class, instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618 raise instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 In the first form, ``instance`` must be an instance of :class:`Class` or of a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	621 class derived from it. The second form is a shorthand for::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	622
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623 raise instance.__class__, instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	624
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	625 A class in an except clause is compatible with an exception if it is the same
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	626 class or a base class thereof (but not the other way around --- an except clause
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	627 listing a derived class is not compatible with a base class). For example, the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	628 following code will print B, C, D in that order::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	629
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	630 class B:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	631 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	632 class C(B):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	633 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	634 class D(C):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	635 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	636
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	637 for c in [B, C, D]:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	638 try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	639 raise c()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	640 except D:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	641 print "D"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	642 except C:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	643 print "C"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	644 except B:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	645 print "B"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	646
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	647 Note that if the except clauses were reversed (with ``except B`` first), it
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	648 would have printed B, B, B --- the first matching except clause is triggered.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	649
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	650 When an error message is printed for an unhandled exception, the exception's
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	651 class name is printed, then a colon and a space, and finally the instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	652 converted to a string using the built-in function :func:`str`.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	653
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	654
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	655 .. _tut-iterators:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	656
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	657 Iterators
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	658 =========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	659
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	660 By now you have probably noticed that most container objects can be looped over
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	661 using a :keyword:`for` statement::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	662
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	663 for element in [1, 2, 3]:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	664 print element
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	665 for element in (1, 2, 3):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	666 print element
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	667 for key in {'one':1, 'two':2}:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	668 print key
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	669 for char in "123":
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	670 print char
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	671 for line in open("myfile.txt"):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	672 print line
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	673
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	674 This style of access is clear, concise, and convenient. The use of iterators
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	675 pervades and unifies Python. Behind the scenes, the :keyword:`for` statement
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	676 calls :func:`iter` on the container object. The function returns an iterator
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	677 object that defines the method :meth:`next` which accesses elements in the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	678 container one at a time. When there are no more elements, :meth:`next` raises a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	679 :exc:`StopIteration` exception which tells the :keyword:`for` loop to terminate.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	680 This example shows how it all works::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	681
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	682 >>> s = 'abc'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	683 >>> it = iter(s)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	684 >>> it
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	685 <iterator object at 0x00A1DB50>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	686 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	687 'a'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	688 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	689 'b'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	690 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	691 'c'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	692 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	693
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	694 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	695 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	696 it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	697 StopIteration
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	698
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	699 Having seen the mechanics behind the iterator protocol, it is easy to add
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	700 iterator behavior to your classes. Define a :meth:`__iter__` method which
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	701 returns an object with a :meth:`next` method. If the class defines
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	702 :meth:`next`, then :meth:`__iter__` can just return ``self``::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	703
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	704 class Reverse:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	705 "Iterator for looping over a sequence backwards"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	706 def __init__(self, data):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	707 self.data = data
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	708 self.index = len(data)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	709 def __iter__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	710 return self
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	711 def next(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	712 if self.index == 0:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	713 raise StopIteration
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	714 self.index = self.index - 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	715 return self.data[self.index]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	716
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	717 >>> for char in Reverse('spam'):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	718 ... print char
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	719 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	720 m
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	721 a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	722 p
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	723 s
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	724
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	725
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	726 .. _tut-generators:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	727
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	728 Generators
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	729 ==========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	730
43629 4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	731 :term:`Generator`\s are a simple and powerful tool for creating iterators. They
4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	732 are written like regular functions but use the :keyword:`yield` statement
4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	733 whenever they want to return data. Each time :meth:`next` is called, the
4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	734 generator resumes where it left-off (it remembers all the data values and which
4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	735 statement was last executed). An example shows that generators can be trivially
4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	736 easy to create::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	737
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	738 def reverse(data):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	739 for index in range(len(data)-1, -1, -1):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	740 yield data[index]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	741
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	742 >>> for char in reverse('golf'):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	743 ... print char
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	744 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	745 f
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	746 l
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	747 o
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	748 g
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	749
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	750 Anything that can be done with generators can also be done with class based
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	751 iterators as described in the previous section. What makes generators so
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	752 compact is that the :meth:`__iter__` and :meth:`next` methods are created
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	753 automatically.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	754
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	755 Another key feature is that the local variables and execution state are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	756 automatically saved between calls. This made the function easier to write and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	757 much more clear than an approach using instance variables like ``self.index``
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	758 and ``self.data``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	759
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	760 In addition to automatic method creation and saving program state, when
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	761 generators terminate, they automatically raise :exc:`StopIteration`. In
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	762 combination, these features make it easy to create iterators with no more effort
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	763 than writing a regular function.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	764
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	765
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	766 .. _tut-genexps:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	767
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	768 Generator Expressions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	769 =====================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	770
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	771 Some simple generators can be coded succinctly as expressions using a syntax
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	772 similar to list comprehensions but with parentheses instead of brackets. These
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	773 expressions are designed for situations where the generator is used right away
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	774 by an enclosing function. Generator expressions are more compact but less
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	775 versatile than full generator definitions and tend to be more memory friendly
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	776 than equivalent list comprehensions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	777
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	778 Examples::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	779
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	780 >>> sum(i*i for i in range(10)) # sum of squares
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	781 285
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	782
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	783 >>> xvec = [10, 20, 30]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	784 >>> yvec = [7, 5, 3]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	785 >>> sum(x*y for x,y in zip(xvec, yvec)) # dot product
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	786 260
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	787
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	788 >>> from math import pi, sin
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	789 >>> sine_table = dict((x, sin(x*pi/180)) for x in range(0, 91))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	790
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	791 >>> unique_words = set(word for line in page for word in line.split())
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	792
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	793 >>> valedictorian = max((student.gpa, student.name) for student in graduates)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	794
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	795 >>> data = 'golf'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	796 >>> list(data[i] for i in range(len(data)-1,-1,-1))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	797 ['f', 'l', 'o', 'g']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	798
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	799
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	800
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	801 .. rubric:: Footnotes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	802
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	803 .. [#] Except for one thing. Module objects have a secret read-only attribute called
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	804 :attr:`__dict__` which returns the dictionary used to implement the module's
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	805 namespace; the name :attr:`__dict__` is an attribute but not a global name.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	806 Obviously, using this violates the abstraction of namespace implementation, and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	807 should be restricted to things like post-mortem debuggers.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	808

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/classes.rst @ 47787:b32101c19bf8 legacy-trunk