\chapter{Execution model \label{execmodel}}

\index{execution model}

\section{Code blocks, execution frames, and namespaces \label{execframes}}

\index{code block}

\index{namespace}

\indexii{execution}{frame}

A \dfn{code block}\indexii{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

`\strong{-c}' option) is a code block. The file read by the built-in

function \function{execfile()} is a code block. The string argument

passed to the built-in function \function{eval()} and to the

\keyword{exec} statement is a code block. And finally, the expression

read and evaluated by the built-in function \function{input()} is a

code block.

A code block is executed in an execution frame. An \dfn{execution

frame}\indexii{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 \dfn{namespace}\index{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 \dfn{binding}\indexii{binding}{name} a

name (to an object); changing the mapping of a name is called

\dfn{rebinding}\indexii{rebinding}{name}; removing a name is

\dfn{unbinding}\indexii{unbinding}{name}. Namespaces are functionally

equivalent to dictionaries (and often implemented as dictionaries).

The \dfn{local namespace}\indexii{local}{namespace} of an execution

frame determines the default place where names are defined and

searched. The

\dfn{global namespace}\indexii{global}{namespace} determines the place

where names listed in \keyword{global}\stindex{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 \keyword{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 \keyword{global} statement forces global

interpretation of selected names throughout the code block. The

following constructs bind names: formal parameters to functions,

\keyword{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, \keyword{for} loop

header, or in the second position of an \keyword{except} clause

header. Local names are searched only on the local namespace; global

names are searched only in the global and built-in

namespace.\footnote{

If the code block contains \keyword{exec} statements or the

construct ``\samp{from \ldots import *}'', the semantics of local

names change: local name lookup first searches the local namespace,

then the global namespace and the built-in namespace.}

A target occurring in a \keyword{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

\module{__builtin__}\refbimodindex{__builtin__}). The built-in

namespace associated with the execution of a code block is actually

found by looking up the name \code{__builtins__} is its global

namespace; this should be a dictionary or a module (in the latter case

its dictionary is used). Normally, the \code{__builtins__} namespace

is the dictionary of the built-in module \module{__builtin__} (note:

no `s'); if it isn't, restricted

execution\indexii{restricted}{execution} mode is in effect. When a

name is not found at all, a

\exception{NameError}\withsubitem{(built-in

exception)}{\ttindex{NameError}} exception is raised.

\stindex{from}

\stindex{exec}

\stindex{global}

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!

\begin{tableiv}{l|l|l|l}{textrm}

{Code block type}{Global namespace}{Local namespace}{Notes}

\lineiv{Module}

{n.s. for this module}

{same as global}{}

\lineiv{Script (file or command)}

{n.s. for \module{__main__}\refbimodindex{__main__}}

{same as global}{(1)}

\lineiv{Interactive command}

{n.s. for \module{__main__}\refbimodindex{__main__}}

{same as global}{}

\lineiv{Class definition}

{global n.s. of containing block}

{new n.s.}{}

\lineiv{Function body}

{global n.s. of containing block}

{new n.s.}{(2)}

\lineiv{String passed to \keyword{exec} statement}

{global n.s. of containing block}

{local n.s. of containing block}{(2), (3)}

\lineiv{String passed to \function{eval()}}

{global n.s. of caller}

{local n.s. of caller}{(2), (3)}

\lineiv{File read by \function{execfile()}}

{global n.s. of caller}

{local n.s. of caller}{(2), (3)}

\lineiv{Expression read by \function{input()}}

{global n.s. of caller}

{local n.s. of caller}{}

\end{tableiv}

Notes:

\begin{description}

\item[n.s.] means \emph{namespace}

\item[(1)] The main module for a script is always called

\module{__main__}; ``the filename don't enter into it.''

\item[(2)] The global and local namespace for these can be

overridden with optional extra arguments.

\item[(3)] The \keyword{exec} statement and the \function{eval()} and

\function{execfile()} functions have optional arguments to override

the global and local namespace. If only one namespace is specified,

it is used for both.

\end{description}

The built-in functions \function{globals()} and \function{locals()} returns a

dictionary representing the current global and local namespace,

respectively. The effect of modifications to this dictionary on the

namespace are undefined.\footnote{

The current implementations return the dictionary actually used to

implement the namespace, \emph{except} for functions, where the

optimizer may cause the local namespace to be implemented

differently, and \function{locals()} returns a read-only

dictionary.}

\section{Exceptions \label{exceptions}}

\index{exception}

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

\emph{raised}\index{raise an exception} at the point where the error

is detected; it may be \emph{handled}\index{handle an exception} by

the surrounding code block or by any code block that directly or

indirectly invoked the code block where the error occurred.

\index{exception handler}

\index{errors}

\index{error handling}

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 \keyword{raise} statement.

Exception handlers are specified with the \keyword{try} ... \keyword{except}

statement. The \keyword{try} ... \keyword{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'' \index{termination model}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 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

either case, it prints a stack backtrace, except when the exception is

\exception{SystemExit}\withsubitem{(built-in

exception)}{\ttindex{SystemExit}}.

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 \keyword{except}

clause must reference the same string object. For class exceptions,

the \keyword{except} clause must reference the same class or a base

class of it.

When an exception is raised, an object (maybe \code{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 \keyword{try} statement in section

\ref{try} and \keyword{raise} statement in section \ref{raise}.