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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 |

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.