A code block is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code block may
textually contain other code blocks. Code blocks may invoke other
code blocks (that may or may not be textually contained in them) as
part of their execution, e.g. by invoking (calling) a function.
The following are code blocks: A module is a code block. A function body is a code block. A class definition is a code block. Each command typed interactively is a separate code block; a script file is a code block. The string argument passed to the built-in function eval and to the exec statement are code blocks. And finally, the expression read and evaluated by the built-in function input is a code block.
A code block is executed in an execution frame. An execution
frame contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
name spaces, the local and the global name space, that affect
execution of the code block.
A name space is a mapping from names (identifiers) to objects.
A particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called binding a name (to an object); changing the mapping of
a name is called rebinding; removing a name is unbinding.
Name spaces are functionally equivalent to dictionaries.
The local name space of an execution frame determines the default
place where names are defined and searched. The global name
space determines the place where names listed in global
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
Whether a name is local or global in a code block is determined by static inspection of the source text for the code block: in the absence of global statements, a name that is bound anywhere in the code block is local in the entire code block; all other names are considered global. The global statement forces global interpretation of selected names throughout the code block. The following constructs bind names: formal parameters, import statements, class and function definitions (these bind the class or function name), and targets that are identifiers if occurring in an assignment, for loop header, or except clause header.
A target occurring in a del statement is also considered bound for this purpose (though the actual semantics are to ``unbind'' the name).
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (which is actually the
global name space of the module __builtin__). When a name is not
found at all, the NameError exception is raised.
The following table lists the meaning of the local and global name space for various types of code blocks. The name space for a particular module is automatically created when the module is first referenced.
[tabular406]
Notes: