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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:: 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.
+
+.. 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:`try` ...
+:keyword:`finally` statement specifies 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.
+