.. _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:: single: block

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

file read by the built-in function :func:`execfile` is a code block.  The string

argument passed to the built-in function :func:`eval` and to the :keyword:`exec`

statement is a code block. The expression read and evaluated by the built-in

function :func:`input` is a code block.

.. 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:: single: scope

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

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))

.. 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. 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`.

.. 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 in the second

position of an :keyword:`except` clause header.  The :keyword:`import` statement

of the form "``from ...import *``" binds all names defined in the imported

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

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 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, and the builtin

namespace, the namespace of the module :mod:`__builtin__`.  The global namespace

is searched first.  If the name is not found there, the builtin namespace is

searched.  The global statement must precede all uses of the name.

.. 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:`__builtin__` (note: no 's'); when in any other module,

``__builtins__`` is an alias for the dictionary of the :mod:`__builtin__` module

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:`__builtin__` (no 's') module and modify its

   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

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`.

If :keyword:`exec` is used in a function and the function contains or is a

nested block with free variables, the compiler will raise a :exc:`SyntaxError`

unless the exec explicitly specifies the local namespace for the

:keyword:`exec`.  (In other words, ``exec obj`` would be illegal, but ``exec obj

in ns`` would be legal.)

The :func:`eval`, :func:`execfile`, and :func:`input` functions and the

:keyword:`exec` statement 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 :keyword:`exec` statement and the

:func:`eval` and :func:`execfile` functions have optional arguments to override

the global and local namespace.  If only one namespace is specified, it is used

for both.

.. _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

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.

.. 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`.

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.

Exceptions can also be identified by strings, in which case the

:keyword:`except` clause is selected by object identity.  An arbitrary value can

be raised along with the identifying string which can be passed to the handler.

.. warning::

   Messages to exceptions 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

   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`.

.. rubric:: Footnotes

.. [#] This limitation occurs because the code that is executed by these operations is

   not available at the time the module is compiled.