201 lines
8.2 KiB
TeX
201 lines
8.2 KiB
TeX
\chapter{Execution model}
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\index{execution model}
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\section{Code blocks, execution frames, and name spaces} \label{execframes}
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\index{code block}
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\indexii{execution}{frame}
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\index{name space}
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A {\em 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 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 is
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a code block. The string argument passed to the built-in function
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\function{eval()} and to the \keyword{exec} statement are code blocks.
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And finally, the expression read and evaluated by the built-in
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function \function{input()} is a code block.
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A code block is executed in an execution frame. An {\em 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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name spaces, the local and the global name space, that affect
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execution of the code block.
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\indexii{execution}{frame}
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A {\em name space} is a mapping from names (identifiers) to objects.
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A particular name space may be referenced by more than one execution
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frame, and from other places as well. Adding a name to a name space
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is called {\em binding} a name (to an object); changing the mapping of
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a name is called {\em rebinding}; removing a name is {\em unbinding}.
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Name spaces are functionally equivalent to dictionaries.
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\index{name space}
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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 {\em local name space} of an execution frame determines the default
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place where names are defined and searched. The {\em global name
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space} 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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explicitly bound in the current code block are searched.
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\indexii{local}{name space}
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\indexii{global}{name space}
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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 in
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the code block is local in the entire code block; all other names are
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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, \keyword{import}
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statements, class and function definitions (these bind the class or
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function name), and targets that are identifiers if occurring in an
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assignment, \keyword{for} loop header, or except clause header.
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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 name space, it is
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searched in the list of ``built-in'' names (which is actually the
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global name space of the module \module{__builtin__}). When a name is not
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found at all, the \exception{NameError} exception is raised.%
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\footnote{If the code block contains \keyword{exec} statements or the
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construct \samp{from \ldots import *}, the semantics of names not
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explicitly mentioned in a {\tt global} statement change subtly: name
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lookup first searches the local name space, then the global one, then
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the built-in one.}
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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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\withsubitem{(built-in exception)}{\ttindex{NameError}}
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The following table lists the meaning of the local and global name
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space for various types of code blocks. The name space for a
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particular module is automatically created when the module is first
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referenced. Note that in almost all cases, the global name space is
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the name space of the containing module --- scopes in Python do not
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nest!
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\begin{center}
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\begin{tabular}{|l|l|l|l|}
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\hline
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Code block type & Global name space & Local name space & Notes \\
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\hline
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Module & n.s. for this module & same as global & \\
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Script & n.s. for \module{__main__} & same as global & \\
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Interactive command & n.s. for \module{__main__} & same as global & \\
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Class definition & global n.s. of containing block & new n.s. & \\
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Function body & global n.s. of containing block & new n.s. & (2) \\
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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 & (1) \\
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String passed to \function{eval()}
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& global n.s. of caller & local n.s. of caller & (1) \\
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File read by \function{execfile()}
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& global n.s. of caller & local n.s. of caller & (1) \\
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Expression read by \function{input()}
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& global n.s. of caller & local n.s. of caller & \\
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\hline
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\end{tabular}
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\end{center}
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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 {\em name space}
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\item[(1)] The global and local name space for these can be
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overridden with optional extra arguments.
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\item[(2)] The body of lambda forms (see section \ref{lambda}) is
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treated exactly the same as a (nested) function definition. Lambda
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forms have their own name space consisting of their formal arguments.
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\indexii{lambda}{form}
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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 name space,
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respectively. The effect of modifications to this dictionary on the
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name space are undefined.%
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\footnote{The current implementations return the dictionary actually
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used to implement the name space, {\em except} for functions, where
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the optimizer may cause the local name space to be implemented
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differently, and \function{locals()} returns a read-only dictionary.}
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\section{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 {\em raised} at the point where the error
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is detected; it may be {\em 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 an 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.
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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 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.
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Exceptions are identified by string objects or class instances. Two
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different string objects with the same value identify different
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exceptions. An exception can be raised with a class instance. Such
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exceptions are caught by specifying an except clause that has the
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class name (or a base class) as the condition.
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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''; this object does not affect the
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selection of an exception handler, but is passed to the selected
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exception handler as additional information. For exceptions raised
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with a class instance, the instance is passed as the ``parameter''.
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For example:
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\begin{verbatim}
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>>> class Error:
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... def __init__(self, msg): self.msg = msg
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...
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>>> class SpecificError(Error): pass
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...
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>>> try:
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... raise SpecificError('broken')
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... except Error, obj:
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... print obj.msg
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...
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broken
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\end{verbatim}
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See also the description of the \keyword{try} and \keyword{raise}
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statements.
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