cpython/Doc/reference/executionmodel.rst

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.. _execmodel:
***************
Execution model
***************
.. index:: single: execution model
.. _naming:
Naming and binding
==================
.. index::
pair: code; block
single: namespace
single: scope
.. index::
single: name
pair: binding; name
:dfn:`Names` refer to objects. Names are introduced by name binding operations.
Each occurrence of a name in the program text refers to the :dfn:`binding` of
that name established in the innermost function block containing the use.
.. index:: block
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A :dfn:`block` is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class definition.
Each command typed interactively is a block. A script file (a file given as
standard input to the interpreter or specified on the interpreter command line
the first argument) is a code block. A script command (a command specified on
the interpreter command line with the '**-c**' option) is a code block. The
string argument passed to the built-in functions :func:`eval` and :func:`exec`
is a code block.
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.. index:: pair: execution; frame
A code block is executed in an :dfn:`execution frame`. A frame contains some
administrative information (used for debugging) and determines where and how
execution continues after the code block's execution has completed.
.. index:: scope
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A :dfn:`scope` defines the visibility of a name within a block. If a local
variable is defined in a block, its scope includes that block. If the
definition occurs in a function block, the scope extends to any blocks contained
within the defining one, unless a contained block introduces a different binding
for the name. The scope of names defined in a class block is limited to the
class block; it does not extend to the code blocks of methods -- this includes
comprehensions and generator expressions since they are implemented using a
function scope. This means that the following will fail::
class A:
a = 42
b = list(a + i for i in range(10))
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.. index:: single: environment
When a name is used in a code block, it is resolved using the nearest enclosing
scope. The set of all such scopes visible to a code block is called the block's
:dfn:`environment`.
.. index:: pair: free; variable
If a name is bound in a block, it is a local variable of that block, unless
declared as :keyword:`nonlocal`. If a name is bound at the module level, it is
a global variable. (The variables of the module code block are local and
global.) If a variable is used in a code block but not defined there, it is a
:dfn:`free variable`.
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.. index::
single: NameError (built-in exception)
single: UnboundLocalError
When a name is not found at all, a :exc:`NameError` exception is raised. If the
name refers to a local variable that has not been bound, a
:exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a
subclass of :exc:`NameError`.
.. index:: statement: from
The following constructs bind names: formal parameters to functions,
:keyword:`import` statements, class and function definitions (these bind the
class or function name in the defining block), and targets that are identifiers
if occurring in an assignment, :keyword:`for` loop header, or after
:keyword:`as` in a :keyword:`with` statement or :keyword.`except` clause.
The :keyword:`import` statement
of the form ``from ... import *`` binds all names defined in the imported
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module, except those beginning with an underscore. This form may only be used
at the module level.
A target occurring in a :keyword:`del` statement is also considered bound for
this purpose (though the actual semantics are to unbind the name). It is
illegal to unbind a name that is referenced by an enclosing scope; the compiler
will report a :exc:`SyntaxError`.
Each assignment or import statement occurs within a block defined by a class or
function definition or at the module level (the top-level code block).
If a name binding operation occurs anywhere within a code block, all uses of the
name within the block are treated as references to the current block. This can
lead to errors when a name is used within a block before it is bound. This rule
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is subtle. Python lacks declarations and allows name binding operations to
occur anywhere within a code block. The local variables of a code block can be
determined by scanning the entire text of the block for name binding operations.
If the :keyword:`global` statement occurs within a block, all uses of the name
specified in the statement refer to the binding of that name in the top-level
namespace. Names are resolved in the top-level namespace by searching the
global namespace, i.e. the namespace of the module containing the code block,
Merged revisions 73941-73943,74076,74094,74186,74211-74214,74247,74254,74262,74311,74334,74368 via svnmerge from svn+ssh://svn.python.org/python/branches/py3k ................ r73941 | georg.brandl | 2009-07-11 12:39:00 +0200 (Sa, 11 Jul 2009) | 9 lines Merged revisions 73940 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r73940 | georg.brandl | 2009-07-11 12:37:38 +0200 (Sa, 11 Jul 2009) | 1 line #6430: add note about size of "u" type. ........ ................ r73942 | georg.brandl | 2009-07-11 12:39:23 +0200 (Sa, 11 Jul 2009) | 1 line #6430: remove mention of "w" array typecode. ................ r73943 | georg.brandl | 2009-07-11 12:43:08 +0200 (Sa, 11 Jul 2009) | 1 line #6421: The self argument of module-level PyCFunctions is now a reference to the module object. ................ r74076 | georg.brandl | 2009-07-18 11:07:48 +0200 (Sa, 18 Jul 2009) | 1 line #6502: add missing comma in docstring. ................ r74094 | georg.brandl | 2009-07-19 09:25:56 +0200 (So, 19 Jul 2009) | 10 lines Recorded merge of revisions 74089 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74089 | senthil.kumaran | 2009-07-19 04:43:43 +0200 (So, 19 Jul 2009) | 3 lines Fix for issue5102, timeout value propages between redirects, proxy, digest and auth handlers. Fixed tests to reflect the same. ........ ................ r74186 | georg.brandl | 2009-07-23 11:19:09 +0200 (Do, 23 Jul 2009) | 9 lines Recorded merge of revisions 74185 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74185 | georg.brandl | 2009-07-23 11:17:09 +0200 (Do, 23 Jul 2009) | 1 line Fix the "pylocals" gdb command. ........ ................ r74211 | georg.brandl | 2009-07-26 16:48:09 +0200 (So, 26 Jul 2009) | 9 lines Recorded merge of revisions 74210 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74210 | georg.brandl | 2009-07-26 16:44:23 +0200 (So, 26 Jul 2009) | 1 line Move member descriptions inside the classes. ........ ................ r74212 | georg.brandl | 2009-07-26 16:54:51 +0200 (So, 26 Jul 2009) | 9 lines Merged revisions 74209 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74209 | georg.brandl | 2009-07-26 16:37:28 +0200 (So, 26 Jul 2009) | 1 line builtin -> built-in. ........ ................ r74213 | georg.brandl | 2009-07-26 17:02:41 +0200 (So, 26 Jul 2009) | 9 lines Merged revisions 74207 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74207 | georg.brandl | 2009-07-26 16:19:57 +0200 (So, 26 Jul 2009) | 1 line #6577: fix (hopefully) all links to builtin instead of module/class-specific objects. ........ ................ r74214 | georg.brandl | 2009-07-26 17:03:49 +0200 (So, 26 Jul 2009) | 9 lines Merged revisions 74205 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74205 | georg.brandl | 2009-07-26 15:36:39 +0200 (So, 26 Jul 2009) | 1 line #6576: fix cross-refs in re docs. ........ ................ r74247 | georg.brandl | 2009-07-29 09:27:08 +0200 (Mi, 29 Jul 2009) | 9 lines Merged revisions 74239 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74239 | georg.brandl | 2009-07-28 18:55:32 +0000 (Di, 28 Jul 2009) | 1 line Clarify quote_plus() usage. ........ ................ r74254 | georg.brandl | 2009-07-29 18:14:16 +0200 (Mi, 29 Jul 2009) | 1 line #6586: fix return/argument type doc for os.read() and os.write(). ................ r74262 | alexandre.vassalotti | 2009-07-29 21:54:39 +0200 (Mi, 29 Jul 2009) | 57 lines Merged revisions 74074,74077,74111,74188,74192-74193,74200,74252-74253,74258-74261 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74074 | georg.brandl | 2009-07-18 05:03:10 -0400 (Sat, 18 Jul 2009) | 1 line #6513: fix example code: warning categories are classes, not instances. ........ r74077 | georg.brandl | 2009-07-18 05:43:40 -0400 (Sat, 18 Jul 2009) | 1 line #6489: fix an ambiguity in getiterator() documentation. ........ r74111 | benjamin.peterson | 2009-07-20 09:30:10 -0400 (Mon, 20 Jul 2009) | 1 line remove docs for deprecated -p option ........ r74188 | benjamin.peterson | 2009-07-23 10:25:31 -0400 (Thu, 23 Jul 2009) | 1 line use bools ........ r74192 | georg.brandl | 2009-07-24 12:28:38 -0400 (Fri, 24 Jul 2009) | 1 line Fix arg types of et#. ........ r74193 | georg.brandl | 2009-07-24 12:46:38 -0400 (Fri, 24 Jul 2009) | 1 line Dont put "void" in signature for nullary functions. ........ r74200 | georg.brandl | 2009-07-25 09:02:15 -0400 (Sat, 25 Jul 2009) | 1 line #6571: add index entries for more operators. ........ r74252 | georg.brandl | 2009-07-29 12:06:31 -0400 (Wed, 29 Jul 2009) | 1 line #6593: fix link targets. ........ r74253 | georg.brandl | 2009-07-29 12:09:17 -0400 (Wed, 29 Jul 2009) | 1 line #6591: add reference to ioctl in fcntl module for platforms other than Windows. ........ r74258 | georg.brandl | 2009-07-29 12:57:05 -0400 (Wed, 29 Jul 2009) | 1 line Add a link to readline, and mention IPython and bpython. ........ r74259 | georg.brandl | 2009-07-29 13:07:21 -0400 (Wed, 29 Jul 2009) | 1 line Fix some markup and small factual glitches found by M. Markert. ........ r74260 | georg.brandl | 2009-07-29 13:15:20 -0400 (Wed, 29 Jul 2009) | 1 line Fix a few markup glitches. ........ r74261 | georg.brandl | 2009-07-29 13:50:25 -0400 (Wed, 29 Jul 2009) | 1 line Rewrite the section about classes a bit; mostly tidbits, and a larger update to the section about "private" variables to reflect the Pythonic consensus better. ........ ................ r74311 | georg.brandl | 2009-08-04 22:29:27 +0200 (Di, 04 Aug 2009) | 1 line Slightly improve buffer-related error message. ................ r74334 | georg.brandl | 2009-08-06 19:51:03 +0200 (Do, 06 Aug 2009) | 1 line #6648: mention surrogateescape handler where all standard handlers are listed. ................ r74368 | georg.brandl | 2009-08-13 09:56:35 +0200 (Do, 13 Aug 2009) | 21 lines Merged revisions 74328,74332-74333,74365 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74328 | georg.brandl | 2009-08-06 17:06:25 +0200 (Do, 06 Aug 2009) | 1 line Fix base keyword arg name for int() and long(). ........ r74332 | georg.brandl | 2009-08-06 19:23:21 +0200 (Do, 06 Aug 2009) | 1 line Fix punctuation and one copy-paste error. ........ r74333 | georg.brandl | 2009-08-06 19:43:55 +0200 (Do, 06 Aug 2009) | 1 line #6658: fix two typos. ........ r74365 | georg.brandl | 2009-08-13 09:48:05 +0200 (Do, 13 Aug 2009) | 1 line #6679: Remove mention that sub supports no flags. ........ ................
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and the builtins namespace, the namespace of the module :mod:`builtins`. The
global namespace is searched first. If the name is not found there, the builtins
namespace is searched. The global statement must precede all uses of the name.
.. XXX document "nonlocal" semantics here
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.. index:: pair: restricted; execution
The built-in namespace associated with the execution of a code block is actually
found by looking up the name ``__builtins__`` in its global namespace; this
should be a dictionary or a module (in the latter case the module's dictionary
is used). By default, when in the :mod:`__main__` module, ``__builtins__`` is
the built-in module :mod:`builtins`; when in any other module,
``__builtins__`` is an alias for the dictionary of the :mod:`builtins` module
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itself. ``__builtins__`` can be set to a user-created dictionary to create a
weak form of restricted execution.
.. note::
Users should not touch ``__builtins__``; it is strictly an implementation
detail. Users wanting to override values in the built-in namespace should
:keyword:`import` the :mod:`builtins` module and modify its
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attributes appropriately.
.. index:: module: __main__
The namespace for a module is automatically created the first time a module is
imported. The main module for a script is always called :mod:`__main__`.
The global statement has the same scope as a name binding operation in the same
block. If the nearest enclosing scope for a free variable contains a global
statement, the free variable is treated as a global.
A class definition is an executable statement that may use and define names.
These references follow the normal rules for name resolution. The namespace of
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the class definition becomes the attribute dictionary of the class. Names
defined at the class scope are not visible in methods.
.. _dynamic-features:
Interaction with dynamic features
---------------------------------
There are several cases where Python statements are illegal when used in
conjunction with nested scopes that contain free variables.
If a variable is referenced in an enclosing scope, it is illegal to delete the
name. An error will be reported at compile time.
If the wild card form of import --- ``import *`` --- is used in a function and
the function contains or is a nested block with free variables, the compiler
will raise a :exc:`SyntaxError`.
.. XXX from * also invalid with relative imports (at least currently)
The :func:`eval` and :func:`exec` functions do not have access to the full
environment for resolving names. Names may be resolved in the local and global
namespaces of the caller. Free variables are not resolved in the nearest
enclosing namespace, but in the global namespace. [#]_ The :func:`exec` and
:func:`eval` functions have optional arguments to override the global and local
namespace. If only one namespace is specified, it is used for both.
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.. _exceptions:
Exceptions
==========
.. index:: single: exception
.. index::
single: raise an exception
single: handle an exception
single: exception handler
single: errors
single: error handling
Exceptions are a means of breaking out of the normal flow of control of a code
block in order to handle errors or other exceptional conditions. An exception
is *raised* at the point where the error is detected; it may be *handled* by the
surrounding code block or by any code block that directly or indirectly invoked
the code block where the error occurred.
The Python interpreter raises an exception when it detects a run-time error
(such as division by zero). A Python program can also explicitly raise an
exception with the :keyword:`raise` statement. Exception handlers are specified
Merged revisions 63119-63128,63130-63131,63133,63135-63144,63146-63148,63151-63152,63155-63165,63167-63176,63181-63186,63188-63189 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r63119 | benjamin.peterson | 2008-05-11 20:41:23 -0400 (Sun, 11 May 2008) | 2 lines #2196 hasattr now allows SystemExit and KeyboardInterrupt to propagate ........ r63122 | benjamin.peterson | 2008-05-11 20:46:49 -0400 (Sun, 11 May 2008) | 2 lines make message slightly more informative, so there's no chance of misunderstanding it ........ r63158 | ronald.oussoren | 2008-05-12 07:24:33 -0400 (Mon, 12 May 2008) | 5 lines Remove references to platform 'mac' The 'mac' platform (that is, os.name == 'mac') was used for the MacOS 9 port, which is no longer supported (as of Python 2.4 IIRC). ........ r63159 | ronald.oussoren | 2008-05-12 07:31:05 -0400 (Mon, 12 May 2008) | 8 lines MacOSX: remove dependency on Carbon package for urllib This patch removes the dependency on the Carbon package from urllib. The mac-specific code for getting proxy configuration is now writting in Python using ctypes and uses the SystemConfiguration framework instead of InternetConfig. Also provides a mac-specific implementation of proxy_bypass. ........ r63162 | eric.smith | 2008-05-12 10:00:01 -0400 (Mon, 12 May 2008) | 1 line Added 'n' presentation type for integers. ........ r63164 | georg.brandl | 2008-05-12 12:26:52 -0400 (Mon, 12 May 2008) | 2 lines #1713041: fix pprint's handling of maximum depth. ........ r63170 | georg.brandl | 2008-05-12 12:53:42 -0400 (Mon, 12 May 2008) | 2 lines Fix parameter name for enumerate(). ........ r63173 | georg.brandl | 2008-05-12 13:01:58 -0400 (Mon, 12 May 2008) | 2 lines #2766: remove code without effect. ........ r63174 | georg.brandl | 2008-05-12 13:04:10 -0400 (Mon, 12 May 2008) | 3 lines #2767: don't clear globs in run() call, since they could be needed in tearDown, which clears them at the end. ........ r63175 | georg.brandl | 2008-05-12 13:14:51 -0400 (Mon, 12 May 2008) | 2 lines #1760: try-except-finally is one statement since PEP 341. ........ r63186 | amaury.forgeotdarc | 2008-05-12 17:30:24 -0400 (Mon, 12 May 2008) | 2 lines Sync code with documentation, and remove Win95 support in winsound module. ........ r63189 | amaury.forgeotdarc | 2008-05-12 18:21:39 -0400 (Mon, 12 May 2008) | 3 lines Adapt test_pyclbr to the new version of urllib.py: The new mac-specific functions must be ignored. ........
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with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally`
clause of such a statement can be used to specify cleanup code which does not
handle the exception, but is executed whether an exception occurred or not in
the preceding code.
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.. index:: single: termination model
Python uses the "termination" model of error handling: an exception handler can
find out what happened and continue execution at an outer level, but it cannot
repair the cause of the error and retry the failing operation (except by
re-entering the offending piece of code from the top).
.. index:: single: SystemExit (built-in exception)
When an exception is not handled at all, the interpreter terminates execution of
the program, or returns to its interactive main loop. In either case, it prints
a stack backtrace, except when the exception is :exc:`SystemExit`.
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Exceptions are identified by class instances. The :keyword:`except` clause is
selected depending on the class of the instance: it must reference the class of
the instance or a base class thereof. The instance can be received by the
handler and can carry additional information about the exceptional condition.
.. note::
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Exception messages are not part of the Python API. Their contents may change
from one version of Python to the next without warning and should not be
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relied on by code which will run under multiple versions of the interpreter.
See also the description of the :keyword:`try` statement in section :ref:`try`
and :keyword:`raise` statement in section :ref:`raise`.
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.. rubric:: Footnotes
.. [#] This limitation occurs because the code that is executed by these operations
is not available at the time the module is compiled.
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