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(python2.1-ref.info)Definitions and rules

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Definitions and rules
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"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 can is executed as a
unit.  The following are blocks: a module, a function body, and a class
definition.

A "scope" defines the visibility of a name within a block.  If a local
variable is defined in a block, it's 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.

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.
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".

The name binding operations are assignment, class and function
definition, import statements, for statements, and except statements.
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.

The previous rule is a 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 `__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.

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.