source: trunk/essentials/dev-lang/python/Doc/ref/ref4.tex@ 3315

\chapter{Execution model \label{execmodel}}
\index{execution model}


\section{Naming and binding \label{naming}}
\indexii{code}{block}
\index{namespace}
\index{scope}

\dfn{Names}\index{name} refer to objects.  Names are introduced by
name binding operations.  Each occurrence of a name in the program
text refers to the \dfn{binding}\indexii{binding}{name} of that name
established in the innermost function block containing the use.

A \dfn{block}\index{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 `\strong{-c}' option) is a code
block.  The file read by the built-in function \function{execfile()}
is a code block.  The string argument passed to the built-in function
\function{eval()} and to the \keyword{exec} statement is a code block.
The expression read and evaluated by the built-in function
\function{input()} is a code block.

A code block is executed in an \dfn{execution
frame}\indexii{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 \dfn{scope}\index{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.

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{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
\dfn{free variable}\indexii{free}{variable}.