
Execution model
***************


Naming and binding
==================

*Names* refer to objects.  Names are introduced by name binding
operations. Each occurrence of a name in the program text refers to
the *binding* of that name established in the innermost function block
containing the use.

A *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 ``eval()`` and ``exec()`` is a code block.

A code block is executed in an *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.

A *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))

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 *environment*.

If a name is bound in a block, it is a local variable of that block,
unless declared as ``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 *free variable*.

When a name is not found at all, a ``NameError`` exception is raised.
If the name refers to a local variable that has not been bound, a
``UnboundLocalError`` exception is raised.  ``UnboundLocalError`` is a
subclass of ``NameError``.

The following constructs bind names: formal parameters to functions,
``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, ``for`` loop header, or
after ``as`` in a ``with`` statement or ``except`` clause. The
``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 ``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 ``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 builtins namespace, the
namespace of the module ``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.

The builtins 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
``__main__`` module, ``__builtins__`` is the built-in module
``builtins``; when in any other module, ``__builtins__`` is an alias
for the dictionary of the ``builtins`` module itself.
``__builtins__`` can be set to a user-created dictionary to create a
weak form of restricted execution.

**CPython implementation detail:** Users should not touch
``__builtins__``; it is strictly an implementation detail.  Users
wanting to override values in the builtins namespace should ``import``
the ``builtins`` module and modify its attributes appropriately.

The namespace for a module is automatically created the first time a
module is imported.  The main module for a script is always called
``__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.


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 ``SyntaxError``.

The ``eval()`` and ``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.  [1]
The ``exec()`` and ``eval()`` functions have optional arguments to
override the global and local namespace.  If only one namespace is
specified, it is used for both.


Exceptions
==========

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 ``raise`` statement. Exception
handlers are specified with the ``try`` ... ``except`` statement.  The
``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.

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).

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
``SystemExit``.

Exceptions are identified by class instances.  The ``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: 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 relied on by code which will run under multiple
  versions of the interpreter.

See also the description of the ``try`` statement in section *The try
statement* and ``raise`` statement in section *The raise statement*.

-[ Footnotes ]-

[1] This limitation occurs because the code that is executed by these
    operations is not available at the time the module is compiled.
