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* lib1.tex (section{Built-in Functions}): added bagof(), lambda(), map()
and reduce(). Repharased apply(). Removed or rephrased references to
exec() (now the exec stmt).
* lib4.tex: posix.exec --> posix.execv
* ref4.tex, ref8.tex, tut.tex: builtin --> __builtin__
* lib3.tex (module string): added atof() and atol(), and ato[fl]_error.
149 lines
6.1 KiB
TeX
149 lines
6.1 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 block 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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\verb\eval\ and to the \verb\exec\ statement are code blocks.
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And finally, the
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expression read and evaluated by the built-in function \verb\input\ is
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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 \verb\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 \verb\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 \verb\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, \verb\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, \verb\for\ loop header, or \verb\except\ clause header.
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(A target occurring in a \verb\del\ statement does not bind a 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 \verb\__builtin__\). When a name is not
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found at all, the \verb\NameError\ exception is raised.
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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.
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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 \verb\__main__\ & same as global & \\
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Interactive command & n.s. for \verb\__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. & \\
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String passed to \verb\exec\ or \verb\eval\
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& global n.s. of caller & local n.s. of caller & (1) \\
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File read by \verb\execfile\
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& global n.s. of caller & local n.s. of caller & (1) \\
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Expression read by \verb\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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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 functions can be
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overridden with optional extra arguments.
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\end{description}
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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 \verb\raise\ statement.
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Exception handlers are specified with the \verb\try...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. Two different string
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objects with the same value identify different exceptions.
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When an exception is raised, an object (maybe \verb\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.
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See also the description of the \verb\try\ and \verb\raise\
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statements.
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