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

annotate Doc/tutorial/classes.rst @ 44825:ef859cd754a7 legacy-trunk

Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :)

author	Christian Heimes <christian@cheimes.de>
date	Sun, 20 Jan 2008 14:28:28 +0000
parents	3625234bf3d2
children	15746bf4a0a6

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
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 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	127 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	128 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	129 namespace in the local scope.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 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	132 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	133 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	134 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	135 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	136 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	137 already determined statically.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139 A special quirk of Python is that assignments always go into the innermost
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140 scope. Assignments do not copy data --- they just bind names to objects. The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141 same is true for deletions: the statement ``del x`` removes the binding of ``x``
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	142 from the namespace referenced by the local scope. In fact, all operations that
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	143 introduce new names use the local scope: in particular, import statements and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	144 function definitions bind the module or function name in the local scope. (The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145 :keyword:`global` statement can be used to indicate that particular variables
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146 live in the global scope.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	147
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149 .. _tut-firstclasses:
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 A First Look at Classes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 =======================
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 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	155 new semantics.
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
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 .. _tut-classdefinition:
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 Class Definition Syntax
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 -----------------------
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 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	164
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165 class ClassName:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166 <statement-1>
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 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170 <statement-N>
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 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	173 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	174 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	175
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176 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	177 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	178 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	179 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	180 methods --- again, this is explained later.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 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	183 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	184 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	185 function here.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 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	188 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	189 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	190 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	191 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	192 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	193 example).
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196 .. _tut-classobjects:
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 Class Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 -------------
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 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	202 instantiation.
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 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	205 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	206 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	207 definition looked like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 class MyClass:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 "A simple example class"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211 i = 12345
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212 def f(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213 return 'hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215 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	216 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	217 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	218 :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	219 the class: ``"A simple example class"``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 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	222 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	223 For example (assuming the above class)::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 x = MyClass()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227 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	228 variable ``x``.
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 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	231 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	232 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	233 :meth:`__init__`, like this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	235 def __init__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 self.data = []
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 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	239 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	240 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	241
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	242 x = MyClass()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	244 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	245 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	246 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	247
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	248 >>> class Complex:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	249 ... def __init__(self, realpart, imagpart):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	250 ... self.r = realpart
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	251 ... self.i = imagpart
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	252 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	253 >>> x = Complex(3.0, -4.5)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	254 >>> x.r, x.i
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	255 (3.0, -4.5)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	256
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	257
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	258 .. _tut-instanceobjects:
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 Instance Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	261 ----------------
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 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	264 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	265 attribute names, data attributes and methods.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	267 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	268 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	269 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	270 ``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	271 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	272
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 x.counter = 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 while x.counter < 10:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275 x.counter = x.counter * 2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 print x.counter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 del x.counter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 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	280 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	281 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	282 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	283 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	284 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	285
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 .. index:: object: method
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 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	289 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	290 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	291 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	292 ``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	293 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	294
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296 .. _tut-methodobjects:
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 Method Objects
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 --------------
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 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	302
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303 x.f()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	305 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	306 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	307 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	308
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	309 xf = x.f
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310 while True:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311 print xf()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313 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	314
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315 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	316 ``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	317 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	318 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	319 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	320
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321 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	322 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	323 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	324 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	325 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	326 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	327
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328 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	329 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	330 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	331 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	332 (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	333 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	334 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	335 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	336 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	337
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	338
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	339 .. _tut-remarks:
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 Random Remarks
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	342 ==============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	343
44383 3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	344 .. 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	345
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	346 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	347 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	348 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	349 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	350 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	351 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	352
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	353 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	354 ("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	355 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	356 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	357 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	358 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	359 extensions to Python written in C.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361 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	362 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	363 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	364 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	365 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	366
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367 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	368 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	369 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	370 glancing through a method.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 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	373 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	374 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	375 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	376 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	377
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378 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	379 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	380 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	381 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	382
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383 # Function defined outside the class
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384 def f1(self, x, y):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385 return min(x, x+y)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 class C:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 f = f1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389 def g(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390 return 'hello world'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 h = g
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 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	394 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	395 :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	396 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	397
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 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	399 argument::
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 class Bag:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402 def __init__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403 self.data = []
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 def add(self, x):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405 self.data.append(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 def addtwice(self, x):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	407 self.add(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408 self.add(x)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410 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	411 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	412 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	413 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	414 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	415 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	416 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	417 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	418 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	419
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 .. _tut-inheritance:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	422
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	423 Inheritance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	424 ===========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	425
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426 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	427 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	428 this::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	429
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 class DerivedClassName(BaseClassName):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431 <statement-1>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	433 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	434 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	435 <statement-N>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 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	438 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	439 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	440 class is defined in another module::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442 class DerivedClassName(modname.BaseClassName):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	444 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	445 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	446 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	447 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	448 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	449
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	450 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	451 ``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	452 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	453 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	454 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	455
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 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	457 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	458 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	459 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	460 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	461
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 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	463 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	464 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	465 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	466 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	467 scope.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470 .. _tut-multiple:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472 Multiple Inheritance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473 --------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	475 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	476 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	477
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478 class DerivedClassName(Base1, Base2, Base3):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	479 <statement-1>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	480 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	481 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	482 .
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	483 <statement-N>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	484
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485 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	486 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	487 :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	488 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	489
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	490 (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	491 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	492 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	493 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	494 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	495 :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	496 inherited attributes of :class:`Base1`.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497
43631 03056365a1b6 Add :term:s for "new-style class". Georg Brandl <georg@python.org> parents: 43629 diff changeset	498 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	499 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	500 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	501 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	502
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503 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	504 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	505 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	506 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	507 :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	508 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	509 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	510 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	511 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	512 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	513 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	514 multiple inheritance. For more detail, see
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515 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	516
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518 .. _tut-private:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 Private Variables
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
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523 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	524 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	525 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	526 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	527 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	528 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	529 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	530 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	531 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	532 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	533
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 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	535 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	536 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	537 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	538 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	539 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	540 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	541 (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	542 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	543
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 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	545 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	546 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	547 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	548 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	549 as when referencing ``__dict__`` directly.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552 .. _tut-odds:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 Odds and Ends
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555 =============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 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	558 "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	559 will do nicely::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 class Employee:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 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	565
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566 # Fill the fields of the record
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	567 john.name = 'John Doe'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 john.dept = 'computer lab'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	569 john.salary = 1000
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 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	572 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	573 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	574 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	575 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	576
44383 3625234bf3d2 Some cleanup in the docs. Georg Brandl <georg@python.org> parents: 43631 diff changeset	577 .. (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	578 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	579 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	580 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	581
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582 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	583 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	584 corresponding to the method.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 .. _tut-exceptionclasses:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589 Exceptions Are Classes Too
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 ==========================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592 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	593 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	594
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595 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	596
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	597 raise Class, instance
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	598
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	599 raise instance
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 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	602 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	603
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	604 raise instance.__class__, instance
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 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	607 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	608 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	609 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	610
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611 class B:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613 class C(B):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 pass
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615 class D(C):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 pass
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 for c in [B, C, D]:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619 try:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 raise c()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	621 except D:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	622 print "D"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623 except C:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	624 print "C"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	625 except B:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	626 print "B"
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	627
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	628 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	629 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	630
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	631 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	632 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	633 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	634
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	635
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	636 .. _tut-iterators:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	637
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	638 Iterators
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	639 =========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	640
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	641 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	642 using a :keyword:`for` statement::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	643
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	644 for element in [1, 2, 3]:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	645 print element
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	646 for element in (1, 2, 3):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	647 print element
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	648 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	649 print key
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	650 for char in "123":
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	651 print char
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	652 for line in open("myfile.txt"):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	653 print line
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 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	656 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	657 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	658 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	659 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	660 :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	661 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	662
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	663 >>> s = 'abc'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	664 >>> it = iter(s)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	665 >>> it
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	666 <iterator object at 0x00A1DB50>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	667 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	668 'a'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	669 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	670 'b'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	671 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	672 'c'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	673 >>> it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	674
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	675 Traceback (most recent call last):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	676 File "<stdin>", line 1, in ?
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	677 it.next()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	678 StopIteration
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	679
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	680 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	681 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	682 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	683 :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	684
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	685 class Reverse:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	686 "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	687 def __init__(self, data):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	688 self.data = data
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	689 self.index = len(data)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	690 def __iter__(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	691 return self
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	692 def next(self):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	693 if self.index == 0:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	694 raise StopIteration
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	695 self.index = self.index - 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	696 return self.data[self.index]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	697
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	698 >>> for char in Reverse('spam'):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	699 ... print char
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	700 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	701 m
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	702 a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	703 p
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	704 s
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	705
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	706
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	707 .. _tut-generators:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	708
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	709 Generators
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	710 ==========
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	711
43629 4f662ba09f1f Add :term: for generators. Georg Brandl <georg@python.org> parents: 42603 diff changeset	712 :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	713 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	714 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	715 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	716 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	717 easy to create::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	718
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	719 def reverse(data):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	720 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	721 yield data[index]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	722
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	723 >>> for char in reverse('golf'):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	724 ... print char
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 f
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	727 l
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	728 o
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	729 g
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	730
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	731 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	732 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	733 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	734 automatically.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	735
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	736 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	737 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	738 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	739 and ``self.data``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	740
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	741 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	742 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	743 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	744 than writing a regular function.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	745
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	746
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	747 .. _tut-genexps:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	748
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	749 Generator Expressions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	750 =====================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	751
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	752 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	753 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	754 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	755 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	756 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	757 than equivalent list comprehensions.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	758
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	759 Examples::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	760
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	761 >>> 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	762 285
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	763
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	764 >>> xvec = [10, 20, 30]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	765 >>> yvec = [7, 5, 3]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	766 >>> 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	767 260
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	768
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	769 >>> from math import pi, sin
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	770 >>> 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	771
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	772 >>> 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	773
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	774 >>> 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	775
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	776 >>> data = 'golf'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	777 >>> 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	778 ['f', 'l', 'o', 'g']
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
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	781
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	782 .. rubric:: Footnotes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	783
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	784 .. [#] 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	785 :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	786 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	787 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	788 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	789

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/classes.rst @ 44825:ef859cd754a7 legacy-trunk