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218 lines
9.6 KiB
TeX
218 lines
9.6 KiB
TeX
\chapter{Execution model\label{execmodel}}
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\index{execution model}
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\section{Code blocks, execution frames, and namespaces\label{execframes}}
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\index{code block}
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\indexii{execution}{frame}
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\index{namespace}
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A \dfn{code block} is a piece of Python program text that can be
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executed as a unit, such as a module, a class definition or a function
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body. Some code blocks (like modules) are normally executed only once, others
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(like function bodies) may be executed many times. Code blocks may
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textually contain other code blocks. Code blocks may invoke other
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code blocks (that may or may not be textually contained in them) as
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part of their execution, e.g., by invoking (calling) a function.
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\index{code block}
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\indexii{code}{block}
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The following are code blocks: A module is a code block. A function
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body is a code block. A class definition is a code block. Each
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command typed interactively is a separate code block; a script file (a
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file given as standard input to the interpreter or specified on the
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interpreter command line the first argument) is a code block; a script
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command (a command specified on the interpreter command line with the
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`\code{-c}' option) is a code block. The file read by the built-in
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function \function{execfile()} is a code block. The string argument
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passed to the built-in function \function{eval()} and to the
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\keyword{exec} statement is a code block. And finally, the expression
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read and evaluated by the built-in function \function{input()} is a
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code block.
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A code block is executed in an execution frame. An \dfn{execution
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frame} contains some administrative information (used for debugging),
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determines where and how execution continues after the code block's
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execution has completed, and (perhaps most importantly) defines two
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namespaces, the local and the global namespace, that affect
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execution of the code block.
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\indexii{execution}{frame}
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A \dfn{namespace} is a mapping from names (identifiers) to objects.
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A particular namespace may be referenced by more than one execution
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frame, and from other places as well. Adding a name to a namespace
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is called \dfn{binding} a name (to an object); changing the mapping of
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a name is called \dfn{rebinding}; removing a name is \dfn{unbinding}.
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Namespaces are functionally equivalent to dictionaries (and often
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implemented as dictionaries).
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\index{namespace}
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\indexii{binding}{name}
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\indexii{rebinding}{name}
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\indexii{unbinding}{name}
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The \dfn{local namespace} of an execution frame determines the default
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place where names are defined and searched. The \dfn{global
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namespace} determines the place where names listed in \keyword{global}
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statements are defined and searched, and where names that are not
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bound anywhere in the current code block are searched.
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\indexii{local}{namespace}
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\indexii{global}{namespace}
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\stindex{global}
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Whether a name is local or global in a code block is determined by
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static inspection of the source text for the code block: in the
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absence of \keyword{global} statements, a name that is bound anywhere
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in the code block is local in the entire code block; all other names
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are considered global. The \keyword{global} statement forces global
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interpretation of selected names throughout the code block. The
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following constructs bind names: formal parameters to functions,
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\keyword{import} statements, class and function definitions (these
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bind the class or function name in the defining block), and targets
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that are identifiers if occurring in an assignment, \keyword{for} loop
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header, or in the second position of an \keyword{except} clause
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header. Local names are searched only on the local namespace; global
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names are searched only in the global and built-in
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namespace.\footnote{
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If the code block contains \keyword{exec} statements or the construct
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``\samp{from \ldots import *}'', the semantics of local names change:
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local name lookup first searches the local namespace, then the global
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namespace and the built-in namespace.}
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A target occurring in a \keyword{del} statement is also considered bound
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for this purpose (though the actual semantics are to ``unbind'' the
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name).
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When a global name is not found in the global namespace, it is
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searched in the built-in namespace (which is actually the global
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namespace of the module \module{__builtin__}). The built-in namespace
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associated with the execution of a code block is actually found by
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looking up the name \code{__builtins__} is its global namespace; this
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should be a dictionary or a module (in the latter case its dictionary
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is used). Normally, the \code{__builtins__} namespace is the
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dictionary of the built-in module \module{__builtin__} (note: no `s');
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if it isn't, restricted execution mode is in effect. When a name is
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not found at all, a \exception{NameError} exception is raised.%
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\refbimodindex{__builtin__}
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\stindex{from}
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\stindex{exec}
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\stindex{global}
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\indexii{restricted}{execution}
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\withsubitem{(built-in exception)}{\ttindex{NameError}}
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The following table lists the meaning of the local and global
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namespace for various types of code blocks. The namespace for a
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particular module is automatically created when the module is first
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imported (i.e., when it is loaded). Note that in almost all cases,
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the global namespace is the namespace of the containing module ---
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scopes in Python do not nest!
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\begin{tableiv}{l|l|l|l}{textrm}
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{Code block type}{Global namespace}{Local namespace}{Notes}
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\lineiv{Module}
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{n.s. for this module}
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{same as global}{}
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\lineiv{Script (file or command)}
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{n.s. for \module{__main__}}
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{same as global}{(1)}
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\lineiv{Interactive command}
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{n.s. for \module{__main__}}
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{same as global}{}
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\lineiv{Class definition}
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{global n.s. of containing block}
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{new n.s.}{}
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\lineiv{Function body}
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{global n.s. of containing block}
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{new n.s.}{(2)}
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\lineiv{String passed to \keyword{exec} statement}
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{global n.s. of containing block}
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{local n.s. of containing block}{(2), (3)}
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\lineiv{String passed to \function{eval()}}
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{global n.s. of caller}
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{local n.s. of caller}{(2), (3)}
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\lineiv{File read by \function{execfile()}}
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{global n.s. of caller}
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{local n.s. of caller}{(2), (3)}
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\lineiv{Expression read by \function{input()}}
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{global n.s. of caller}
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{local n.s. of caller}{}
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\end{tableiv}
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\refbimodindex{__main__}
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Notes:
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\begin{description}
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\item[n.s.] means \emph{namespace}
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\item[(1)] The main module for a script is always called
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\module{__main__}; ``the filename don't enter into it.''
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\item[(2)] The global and local namespace for these can be
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overridden with optional extra arguments.
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\item[(3)] The \keyword{exec} statement and the \function{eval()} and
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\function{execfile()} functions have optional arguments to override
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the global and local namespace. If only one namespace is specified,
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it is used for both.
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\end{description}
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The built-in functions \function{globals()} and \function{locals()} returns a
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dictionary representing the current global and local namespace,
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respectively. The effect of modifications to this dictionary on the
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namespace are undefined.\footnote{
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The current implementations return the dictionary actually used to
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implement the namespace, \emph{except} for functions, where the
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optimizer may cause the local namespace to be implemented differently,
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and \function{locals()} returns a read-only dictionary.}
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\section{Exceptions\label{exceptions}}
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Exceptions are a means of breaking out of the normal flow of control
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of a code block in order to handle errors or other exceptional
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conditions. An exception is \emph{raised} at the point where the error
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is detected; it may be \emph{handled} by the surrounding code block or
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by any code block that directly or indirectly invoked the code block
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where the error occurred.
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\index{exception}
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\index{raise an exception}
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\index{handle an exception}
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\index{exception handler}
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\index{errors}
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\index{error handling}
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The Python interpreter raises an exception when it detects a run-time
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error (such as division by zero). A Python program can also
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explicitly raise an exception with the \keyword{raise} statement.
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Exception handlers are specified with the \keyword{try} ... \keyword{except}
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statement. The \keyword{try} ... \keyword{finally} statement
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specifies cleanup code which does not handle the exception, but is
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executed whether an exception occurred or not in the preceding code.
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Python uses the ``termination'' model of error handling: an exception
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handler can find out what happened and continue execution at an outer
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level, but it cannot repair the cause of the error and retry the
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failing operation (except by re-entering the offending piece of
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code from the top).
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When an exception is not handled at all, the interpreter terminates
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execution of the program, or returns to its interactive main loop. In
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either case, it prints a stack backtrace, except when the exception is
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\exception{SystemExit}.\ttindex{SystemExit}
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Exceptions are identified by string objects or class instances.
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Selection of a matching except clause is based on object identity
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(i.e., two different string objects with the same value represent
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different exceptions!) For string exceptions, the \keyword{except}
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clause must reference the same string object. For class exceptions,
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the \keyword{except} clause must reference the same class or a base
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class of it.
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When an exception is raised, an object (maybe \code{None}) is passed
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as the exception's ``parameter'' or ``value''; this object does not
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affect the selection of an exception handler, but is passed to the
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selected exception handler as additional information. For class
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exceptions, this object must be an instance of the exception class
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being raised.
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See also the description of the \keyword{try} and \keyword{raise}
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statements in chapter 7.
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