GNU Info

(python2.1-ref.info)Code blocks

---

Next: Exceptions Prev: Execution model Up: Execution model

---

Enter node , (file) or (file)node

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

---------- Footnotes ----------

(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.

(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.