mirror of https://github.com/python/cpython.git
215 lines
9.7 KiB
TeX
215 lines
9.7 KiB
TeX
\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__} in 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}.
|