cpython-lru_cache-weakref: Doc/reference/executionmodel.rst annotate

annotate Doc/reference/executionmodel.rst @ 87690:66c1c9f32567 default tip

Add use_weakref option to functools.lru_cache The use_weakref option makes lru_cache create weak references to the function arguments when creating the cache key, which prevents them from staying in the memory forever even after all other references to them are gone. This is a proof of concept for http://bugs.python.org/issue19859

author	Radomir Dopieralski <devel@sheep.art.pl>
date	Tue, 03 Dec 2013 16:02:48 +0100
parents	10256bd68787
children

rev	line source
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2 .. _execmodel:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 Execution model
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 .. index:: single: execution model
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11 .. _naming:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13 Naming and binding
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16 .. index::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17 pair: code; block
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18 single: namespace
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19 single: scope
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	20
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21 .. index::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 single: name
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23 pair: binding; name
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 :dfn:`Names` refer to objects. Names are introduced by name binding operations.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 Each occurrence of a name in the program text refers to the :dfn:`binding` of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27 that name established in the innermost function block containing the use.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	29 .. index:: block
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31 A :dfn:`block` is a piece of Python program text that is executed as a unit.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32 The following are blocks: a module, a function body, and a class definition.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	33 Each command typed interactively is a block. A script file (a file given as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34 standard input to the interpreter or specified on the interpreter command line
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35 the first argument) is a code block. A script command (a command specified on
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	36 the interpreter command line with the '-c' option) is a code block. The
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	37 string argument passed to the built-in functions :func:`eval` and :func:`exec`
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	38 is a code block.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40 .. index:: pair: execution; frame
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42 A code block is executed in an :dfn:`execution frame`. A frame contains some
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 administrative information (used for debugging) and determines where and how
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 execution continues after the code block's execution has completed.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	46 .. index:: scope
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48 A :dfn:`scope` defines the visibility of a name within a block. If a local
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49 variable is defined in a block, its scope includes that block. If the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 definition occurs in a function block, the scope extends to any blocks contained
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 within the defining one, unless a contained block introduces a different binding
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 for the name. The scope of names defined in a class block is limited to the
44748 4a67673e3481 Merged revisions 60043-60052 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44098 diff changeset	53 class block; it does not extend to the code blocks of methods -- this includes
51201 02a90dc0c29a comprehrensions now have proper scoping #5106 Benjamin Peterson <benjamin@python.org> parents: 48543 diff changeset	54 comprehensions and generator expressions since they are implemented using a
02a90dc0c29a comprehrensions now have proper scoping #5106 Benjamin Peterson <benjamin@python.org> parents: 48543 diff changeset	55 function scope. This means that the following will fail::
44748 4a67673e3481 Merged revisions 60043-60052 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44098 diff changeset	56
4a67673e3481 Merged revisions 60043-60052 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44098 diff changeset	57 class A:
4a67673e3481 Merged revisions 60043-60052 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44098 diff changeset	58 a = 42
4a67673e3481 Merged revisions 60043-60052 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44098 diff changeset	59 b = list(a + i for i in range(10))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 .. index:: single: environment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63 When a name is used in a code block, it is resolved using the nearest enclosing
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64 scope. The set of all such scopes visible to a code block is called the block's
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	65 :dfn:`environment`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 .. index:: pair: free; variable
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	69 If a name is bound in a block, it is a local variable of that block, unless
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	70 declared as :keyword:`nonlocal`. If a name is bound at the module level, it is
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	71 a global variable. (The variables of the module code block are local and
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	72 global.) If a variable is used in a code block but not defined there, it is a
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	73 :dfn:`free variable`.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75 .. index::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 single: NameError (built-in exception)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77 single: UnboundLocalError
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 When a name is not found at all, a :exc:`NameError` exception is raised. If the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 name refers to a local variable that has not been bound, a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81 :exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 subclass of :exc:`NameError`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 .. index:: statement: from
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 The following constructs bind names: formal parameters to functions,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 :keyword:`import` statements, class and function definitions (these bind the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 class or function name in the defining block), and targets that are identifiers
52182 0be1c1f127e0 Merged revisions 70397 via svnmerge from Georg Brandl <georg@python.org> parents: 51201 diff changeset	89 if occurring in an assignment, :keyword:`for` loop header, or after
65139 9afed70d3aca Fix errors found by "make suspicious". Georg Brandl <georg@python.org> parents: 59617 diff changeset	90 :keyword:`as` in a :keyword:`with` statement or :keyword:`except` clause.
52182 0be1c1f127e0 Merged revisions 70397 via svnmerge from Georg Brandl <georg@python.org> parents: 51201 diff changeset	91 The :keyword:`import` statement
0be1c1f127e0 Merged revisions 70397 via svnmerge from Georg Brandl <georg@python.org> parents: 51201 diff changeset	92 of the form ``from ... import *`` binds all names defined in the imported
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 module, except those beginning with an underscore. This form may only be used
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 at the module level.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 A target occurring in a :keyword:`del` statement is also considered bound for
68183 10256bd68787 this isn't true anymore Benjamin Peterson <benjamin@python.org> parents: 67403 diff changeset	97 this purpose (though the actual semantics are to unbind the name).
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 Each assignment or import statement occurs within a block defined by a class or
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 function definition or at the module level (the top-level code block).
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102 If a name binding operation occurs anywhere within a code block, all uses of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 name within the block are treated as references to the current block. This can
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	104 lead to errors when a name is used within a block before it is bound. This rule
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 is subtle. Python lacks declarations and allows name binding operations to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 occur anywhere within a code block. The local variables of a code block can be
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107 determined by scanning the entire text of the block for name binding operations.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	109 If the :keyword:`global` statement occurs within a block, all uses of the name
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	110 specified in the statement refer to the binding of that name in the top-level
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	111 namespace. Names are resolved in the top-level namespace by searching the
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	112 global namespace, i.e. the namespace of the module containing the code block,
55580 7b495c8f2a24 Merged revisions 73941-73943,74076,74094,74186,74211-74214,74247,74254,74262,74311,74334,74368 via svnmerge from Georg Brandl <georg@python.org> parents: 53511 diff changeset	113 and the builtins namespace, the namespace of the module :mod:`builtins`. The
7b495c8f2a24 Merged revisions 73941-73943,74076,74094,74186,74211-74214,74247,74254,74262,74311,74334,74368 via svnmerge from Georg Brandl <georg@python.org> parents: 53511 diff changeset	114 global namespace is searched first. If the name is not found there, the builtins
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	115 namespace is searched. The global statement must precede all uses of the name.
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	116
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	117 .. XXX document "nonlocal" semantics here
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 .. index:: pair: restricted; execution
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120
59617 e7b5fc8646ab Merged revisions 78760,78771-78773,78802,78922,78952 via svnmerge from Georg Brandl <georg@python.org> parents: 56894 diff changeset	121 The builtins namespace associated with the execution of a code block is actually
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 found by looking up the name ``__builtins__`` in its global namespace; this
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 should be a dictionary or a module (in the latter case the module's dictionary
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 is used). By default, when in the :mod:`__main__` module, ``__builtins__`` is
44098 0889daf6d0d9 #1535: rename __builtin__ module to builtins. Georg Brandl <georg@python.org> parents: 43282 diff changeset	125 the built-in module :mod:`builtins`; when in any other module,
0889daf6d0d9 #1535: rename __builtin__ module to builtins. Georg Brandl <georg@python.org> parents: 43282 diff changeset	126 ``__builtins__`` is an alias for the dictionary of the :mod:`builtins` module
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 itself. ``__builtins__`` can be set to a user-created dictionary to create a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 weak form of restricted execution.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129
56894 b085fd0611de Merged revisions 75797 via svnmerge from Georg Brandl <georg@python.org> parents: 55580 diff changeset	130 .. impl-detail::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132 Users should not touch ``__builtins__``; it is strictly an implementation
59617 e7b5fc8646ab Merged revisions 78760,78771-78773,78802,78922,78952 via svnmerge from Georg Brandl <georg@python.org> parents: 56894 diff changeset	133 detail. Users wanting to override values in the builtins namespace should
44098 0889daf6d0d9 #1535: rename __builtin__ module to builtins. Georg Brandl <georg@python.org> parents: 43282 diff changeset	134 :keyword:`import` the :mod:`builtins` module and modify its
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135 attributes appropriately.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137 .. index:: module: __main__
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139 The namespace for a module is automatically created the first time a module is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140 imported. The main module for a script is always called :mod:`__main__`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141
67403 a3288a4e2ead Fix various issues (mostly Python 2 relics) found by Jacques Ducasse. Georg Brandl <georg@python.org> parents: 65139 diff changeset	142 The :keyword:`global` statement has the same scope as a name binding operation
a3288a4e2ead Fix various issues (mostly Python 2 relics) found by Jacques Ducasse. Georg Brandl <georg@python.org> parents: 65139 diff changeset	143 in the same block. If the nearest enclosing scope for a free variable contains
a3288a4e2ead Fix various issues (mostly Python 2 relics) found by Jacques Ducasse. Georg Brandl <georg@python.org> parents: 65139 diff changeset	144 a global statement, the free variable is treated as a global.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146 A class definition is an executable statement that may use and define names.
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	147 These references follow the normal rules for name resolution. The namespace of
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148 the class definition becomes the attribute dictionary of the class. Names
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149 defined at the class scope are not visible in methods.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 .. _dynamic-features:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 Interaction with dynamic features
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155 ---------------------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 There are several cases where Python statements are illegal when used in
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 conjunction with nested scopes that contain free variables.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160 If a variable is referenced in an enclosing scope, it is illegal to delete the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 name. An error will be reported at compile time.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	162
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 If the wild card form of import --- ``import *`` --- is used in a function and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 the function contains or is a nested block with free variables, the compiler
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165 will raise a :exc:`SyntaxError`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	167 .. XXX from * also invalid with relative imports (at least currently)
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	168
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	169 The :func:`eval` and :func:`exec` functions do not have access to the full
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	170 environment for resolving names. Names may be resolved in the local and global
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	171 namespaces of the caller. Free variables are not resolved in the nearest
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	172 enclosing namespace, but in the global namespace. [#]_ The :func:`exec` and
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	173 :func:`eval` functions have optional arguments to override the global and local
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	174 namespace. If only one namespace is specified, it is used for both.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 .. _exceptions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	178
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	179 Exceptions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180 ==========
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 .. index:: single: exception
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	183
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	184 .. index::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 single: raise an exception
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 single: handle an exception
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 single: exception handler
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	188 single: errors
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	189 single: error handling
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 Exceptions are a means of breaking out of the normal flow of control of a code
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192 block in order to handle errors or other exceptional conditions. An exception
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 is raised at the point where the error is detected; it may be handled by the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194 surrounding code block or by any code block that directly or indirectly invoked
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195 the code block where the error occurred.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197 The Python interpreter raises an exception when it detects a run-time error
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198 (such as division by zero). A Python program can also explicitly raise an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 exception with the :keyword:`raise` statement. Exception handlers are specified
47080 58a0d5524f9a Merged revisions 63119-63128,63130-63131,63133,63135-63144,63146-63148,63151-63152,63155-63165,63167-63176,63181-63186,63188-63189 via svnmerge from Alexandre Vassalotti <alexandre@peadrop.com> parents: 44748 diff changeset	200 with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally`
58a0d5524f9a Merged revisions 63119-63128,63130-63131,63133,63135-63144,63146-63148,63151-63152,63155-63165,63167-63176,63181-63186,63188-63189 via svnmerge from Alexandre Vassalotti <alexandre@peadrop.com> parents: 44748 diff changeset	201 clause of such a statement can be used to specify cleanup code which does not
58a0d5524f9a Merged revisions 63119-63128,63130-63131,63133,63135-63144,63146-63148,63151-63152,63155-63165,63167-63176,63181-63186,63188-63189 via svnmerge from Alexandre Vassalotti <alexandre@peadrop.com> parents: 44748 diff changeset	202 handle the exception, but is executed whether an exception occurred or not in
58a0d5524f9a Merged revisions 63119-63128,63130-63131,63133,63135-63144,63146-63148,63151-63152,63155-63165,63167-63176,63181-63186,63188-63189 via svnmerge from Alexandre Vassalotti <alexandre@peadrop.com> parents: 44748 diff changeset	203 the preceding code.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	205 .. index:: single: termination model
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	207 Python uses the "termination" model of error handling: an exception handler can
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 find out what happened and continue execution at an outer level, but it cannot
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 repair the cause of the error and retry the failing operation (except by
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 re-entering the offending piece of code from the top).
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212 .. index:: single: SystemExit (built-in exception)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 When an exception is not handled at all, the interpreter terminates execution of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215 the program, or returns to its interactive main loop. In either case, it prints
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	216 a stack backtrace, except when the exception is :exc:`SystemExit`.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 Exceptions are identified by class instances. The :keyword:`except` clause is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219 selected depending on the class of the instance: it must reference the class of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 the instance or a base class thereof. The instance can be received by the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 handler and can carry additional information about the exceptional condition.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222
53511 11a5452e3305 Merged revisions 72009 via svnmerge from Georg Brandl <georg@python.org> parents: 52182 diff changeset	223 .. note::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	225 Exception messages are not part of the Python API. Their contents may change
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	226 from one version of Python to the next without warning and should not be
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227 relied on by code which will run under multiple versions of the interpreter.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	229 See also the description of the :keyword:`try` statement in section :ref:`try`
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 and :keyword:`raise` statement in section :ref:`raise`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231
48543 89f5cfec0573 #3113: document exception chaining. Georg Brandl <georg@python.org> parents: 47080 diff changeset	232
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	233 .. rubric:: Footnotes
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234
43282 06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	235 .. [#] This limitation occurs because the code that is executed by these operations
06f6682b2166 Continue going through the language reference, bringing it up-to-date. Georg Brandl <georg@python.org> parents: 42604 diff changeset	236 is not available at the time the module is compiled.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	237

Mercurial > cpython-lru_cache-weakref

annotate Doc/reference/executionmodel.rst @ 87690:66c1c9f32567 default tip