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Python 1.5 Reference Manual

# Chapter 4: Execution model

## 4.1 Code blocks, execution frames, and name spaces

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 string argument passed to the built-in function eval and to the exec statement are code blocks. The file read by the built-in function `execfile` 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 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 (and often implemented as 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 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 specified 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 name space; 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 name space, it is searched in the built-in namespace. The built-in namespace associated with the execution of a code block is actually found by looking up the name `__builtins__` is its global name space; 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, see [Ref:XXX]. When a name is not found at all, a NameError exception is raised.

The following table lists the local and global name space used for all types of code blocks. The name space for a particular module is automatically created when the module is first imported. Note that in almost all cases, the global name space is the name space of the containing module -- scopes in Python do not nest!

Name Spaces for Various Code Blocks
Code block type | Global name space | Local name space | 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. |
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 name space

(1) The main module for a script is always called `__main__`; ''the filename don't enter into it.''

(2) The global and local name space 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 name space, respectively. The effect of modifications to this dictionary on the name space are undefined.[2]

## 4.2 Exceptions

Exceptions are a means of breaking out of the normal flow of control of a code block in order to handle errors or other exceptional conditions. An exception is raised at the point where the error is detected; it may be handled by the surrounding code block or by any code block that directly or indirectly invoked the code block where the error occurred.

The Python interpreter raises an exception when it detects a run-time error (such as division by zero). A Python program can also explicitly raise an exception with the raise statement. Exception handlers are specified with the try...except statement. The `try`...`finally` statement specifies cleanup code which does not handle the exception, but is executed whether an exception occurred or not in the preceding code.

Python uses the "termination" model of error handling: an exception handler can find out what happened and continue execution at an outer level, but it cannot repair the cause of the error and retry the failing operation (except by re-entering the the offending piece of code from the top).

When an exception is not handled at all, the interpreter terminates execution of the program, or returns to its interactive main loop. In this case, the interpreter normally prints a stack backtrace.

Exceptions are identified by string objects or class instances. Selection of a matching `except` clause is based on object identity (i.e. two different string objects with the same value represent different exceptions). For string exceptions, the except clause must reference the same string object. For class exceptions, the except clause must reference the same class or a base class of it.

When an exception is raised, an object (maybe None) is passed as the exception's "parameter" or ''value''; this object does not affect the selection of an exception handler, but is passed to the selected exception handler as additional information. For class exceptions, this object must be an instance of the exception class being raised.

See also the description of the try and raise statements in "Compound statements" on page 47.

[1] If the code block contains exec statements or the construct ''from ... import *'', the semantics of local names change subtly: local name lookup first searches in the local namespace, then in the global namespace and in the built-in namespace.

[2] The current implementations return the dictionary actually used to implement the name space, except for functions, where the optimizer may cause the local name space to be implemented differently, and locals() returns a dictionary that is a shadow copy of the actual local name space.

4.1 - Code blocks, execution frames, and name spaces
4.2 - Exceptions

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