4.1 Code blocks, execution frames, and namespaces

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 normally executed only once, others (like function bodies) may be executed many times. Code blocks 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 (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 execfile() is a code block. The string argument passed to the built-in function eval() and to the exec statement is a code block. 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 namespaces, the local and the global namespace, that affect execution of the code block.

A namespace is a mapping from names (identifiers) to objects. A particular namespace may be referenced by more than one execution frame, and from other places as well. Adding a name to a namespace is called binding a name (to an object); changing the mapping of a name is called rebinding; removing a name is unbinding. Namespaces are functionally equivalent to dictionaries (and often implemented as dictionaries).

The local namespace of an execution frame determines the default place where names are defined and searched. The global namespace determines the place where names listed in global statements are defined and searched, and where names that are not bound anywhere 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 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 in the second position of an except clause header. Local names are searched only on the local namespace; global names are searched only in the global and built-in namespace.^4.1

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 namespace, it is searched in the built-in namespace (which is actually the global namespace of the module __builtin__). The built-in namespace associated with the execution of a code block is actually found by looking up the name __builtins__ is its global namespace; this should be a dictionary or a module (in the latter case its dictionary is used). Normally, the __builtins__ namespace is the dictionary of the built-in module __builtin__ (note: no `s'); if it isn't, restricted execution mode is in effect. When a name is not found at all, a NameError exception is raised.

The following table lists the meaning of the local and global namespace for various types of code blocks. The namespace for a particular module is automatically created when the module is first imported (i.e., when it is loaded). Note that in almost all cases, the global namespace is the namespace of the containing module -- scopes in Python do not nest!

Code block type Global namespace Local namespace Notes

Module n.s. for this module same as global
Script (file or command) n.s. for __main__ same as global (1)
Interactive command n.s. for __main__ same as global
Class definition global n.s. of containing block new n.s.
Function body global n.s. of containing block new n.s. (2)
String passed to exec statement global n.s. of containing block local n.s. of containing block (2), (3)
String passed to eval() global n.s. of caller local n.s. of caller (2), (3)
File read by execfile() global n.s. of caller local n.s. of caller (2), (3)
Expression read by input() global n.s. of caller local n.s. of caller

Code block type	Global namespace	Local namespace	Notes
Module	n.s. for this module	same as global
Script (file or command)	n.s. for `__main__`	same as global	(1)
Interactive command	n.s. for `__main__`	same as global
Class definition	global n.s. of containing block	new n.s.
Function body	global n.s. of containing block	new n.s.	(2)
String passed to `exec` statement	global n.s. of containing block	local n.s. of containing block	(2), (3)
String passed to `eval()`	global n.s. of caller	local n.s. of caller	(2), (3)
File read by `execfile()`	global n.s. of caller	local n.s. of caller	(2), (3)
Expression read by `input()`	global n.s. of caller	local n.s. of caller

Notes:

n.s.: means namespace
(1): The main module for a script is always called __main__; ``the filename don't enter into it.''
(2): The global and local namespace for these can be overridden with optional extra arguments.
(3): The exec statement and the eval() and execfile() functions have optional arguments to override the global and local namespace. If only one namespace is specified, it is used for both.

The built-in functions globals() and locals() returns a dictionary representing the current global and local namespace, respectively. The effect of modifications to this dictionary on the namespace are undefined.^4.2

Footnotes

... namespace.^4.1: If the code block contains exec statements or the construct ``"from ...import *"'', the semantics of local names change: local name lookup first searches the local namespace, then the global namespace and the built-in namespace.
... undefined.^4.2: The current implementations return the dictionary actually used to implement the namespace, except for functions, where the optimizer may cause the local namespace to be implemented differently, and locals() returns a read-only dictionary.

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