\chapter{Compound statements\label{compound}}\indexii{compound}{statement} Compound statements contain (groups of) other statements; they affector control the execution of those other statements in some way. Ingeneral, compound statements span multiple lines, although in simpleincarnations a whole compound statement may be contained in one line. The \keyword{if}, \keyword{while} and \keyword{for} statements implementtraditional control flow constructs. \keyword{try} specifies exceptionhandlers and/or cleanup code for a group of statements. Function andclass definitions are also syntactically compound statements. Compound statements consist of one or more `clauses.' A clauseconsists of a header and a `suite.' The clause headers of aparticular compound statement are all at the same indentation level.Each clause header begins with a uniquely identifying keyword and endswith a colon. A suite is a group of statements controlled by aclause. A suite can be one or more semicolon-separated simplestatements on the same line as the header, following the header'scolon, or it can be one or more indented statements on subsequentlines. Only the latter form of suite can contain nested compoundstatements; the following is illegal, mostly because it wouldn't beclear to which \keyword{if} clause a following \keyword{else} clause wouldbelong:\index{clause}\index{suite} \begin{verbatim}if test1: if test2: print x\end{verbatim} Also note that the semicolon binds tighter than the colon in thiscontext, so that in the following example, either all or none of the\keyword{print} statements are executed: \begin{verbatim}if x < y < z: print x; print y; print z\end{verbatim} Summarizing: \begin{productionlist} \production{compound_stmt} {\token{if_stmt}} \productioncont{| \token{while_stmt}} \productioncont{| \token{for_stmt}} \productioncont{| \token{try_stmt}} \productioncont{| \token{with_stmt}} \productioncont{| \token{funcdef}} \productioncont{| \token{classdef}} \production{suite} {\token{stmt_list} NEWLINE | NEWLINE INDENT \token{statement}+ DEDENT} \production{statement} {\token{stmt_list} NEWLINE | \token{compound_stmt}} \production{stmt_list} {\token{simple_stmt} (";" \token{simple_stmt})* [";"]}\end{productionlist} Note that statements always end in a\code{NEWLINE}\index{NEWLINE token} possibly followed by a\code{DEDENT}.\index{DEDENT token} Also note that optionalcontinuation clauses always begin with a keyword that cannot start astatement, thus there are no ambiguities (the `dangling\keyword{else}' problem is solved in Python by requiring nested\keyword{if} statements to be indented).\indexii{dangling}{else} The formatting of the grammar rules in the following sections placeseach clause on a separate line for clarity. \section{The \keyword{if} statement\label{if}}\stindex{if} The \keyword{if} statement is used for conditional execution: \begin{productionlist} \production{if_stmt} {"if" \token{expression} ":" \token{suite}} \productioncont{( "elif" \token{expression} ":" \token{suite} )*} \productioncont{["else" ":" \token{suite}]}\end{productionlist} It selects exactly one of the suites by evaluating the expressions oneby one until one is found to be true (see section~\ref{Booleans} forthe definition of true and false); then that suite is executed (and noother part of the \keyword{if} statement is executed or evaluated). Ifall expressions are false, the suite of the \keyword{else} clause, ifpresent, is executed.\kwindex{elif}\kwindex{else} \section{The \keyword{while} statement\label{while}}\stindex{while}\indexii{loop}{statement} The \keyword{while} statement is used for repeated execution as longas an expression is true: \begin{productionlist} \production{while_stmt} {"while" \token{expression} ":" \token{suite}} \productioncont{["else" ":" \token{suite}]}\end{productionlist} This repeatedly tests the expression and, if it is true, executes thefirst suite; if the expression is false (which may be the first time itis tested) the suite of the \keyword{else} clause, if present, isexecuted and the loop terminates.\kwindex{else} A \keyword{break} statement executed in the first suite terminates theloop without executing the \keyword{else} clause's suite. A\keyword{continue} statement executed in the first suite skips the restof the suite and goes back to testing the expression.\stindex{break}\stindex{continue} \section{The \keyword{for} statement\label{for}}\stindex{for}\indexii{loop}{statement} The \keyword{for} statement is used to iterate over the elements of asequence (such as a string, tuple or list) or other iterable object:\obindex{sequence} \begin{productionlist} \production{for_stmt} {"for" \token{target_list} "in" \token{expression_list} ":" \token{suite}} \productioncont{["else" ":" \token{suite}]}\end{productionlist} The expression list is evaluated once; it should yield an iterableobject. An iterator is created for the result of the{}\code{expression_list}. The suite is then executed once for eachitem provided by the iterator, in theorder of ascending indices. Each item in turn is assigned to thetarget list using the standard rules for assignments, and then thesuite is executed. When the items are exhausted (which is immediatelywhen the sequence is empty), the suite in the \keyword{else} clause, ifpresent, is executed, and the loop terminates.\kwindex{in}\kwindex{else}\indexii{target}{list} A \keyword{break} statement executed in the first suite terminates theloop without executing the \keyword{else} clause's suite. A\keyword{continue} statement executed in the first suite skips the restof the suite and continues with the next item, or with the \keyword{else}clause if there was no next item.\stindex{break}\stindex{continue} The suite may assign to the variable(s) in the target list; this doesnot affect the next item assigned to it. The target list is not deleted when the loop is finished, but if thesequence is empty, it will not have been assigned to at all by theloop. Hint: the built-in function \function{range()} returns asequence of integers suitable to emulate the effect of Pascal's\code{for i := a to b do};e.g., \code{range(3)} returns the list \code{[0, 1, 2]}.\bifuncindex{range}\indexii{Pascal}{language} \warning{There is a subtlety when the sequence is being modifiedby the loop (this can only occur for mutable sequences, i.e. lists).An internal counter is used to keep track of which item is used next,and this is incremented on each iteration. When this counter hasreached the length of the sequence the loop terminates. This means thatif the suite deletes the current (or a previous) item from thesequence, the next item will be skipped (since it gets the index ofthe current item which has already been treated). Likewise, if thesuite inserts an item in the sequence before the current item, thecurrent item will be treated again the next time through the loop.This can lead to nasty bugs that can be avoided by making a temporarycopy using a slice of the whole sequence, e.g.,\index{loop!over mutable sequence}\index{mutable sequence!loop over}} \begin{verbatim}for x in a[:]: if x < 0: a.remove(x)\end{verbatim} \section{The \keyword{try} statement\label{try}}\stindex{try} The \keyword{try} statement specifies exception handlers and/or cleanupcode for a group of statements: \begin{productionlist} \production{try_stmt} {try1_stmt | try2_stmt} \production{try1_stmt} {"try" ":" \token{suite}} \productioncont{("except" [\token{expression} ["," \token{target}]] ":" \token{suite})+} \productioncont{["else" ":" \token{suite}]} \productioncont{["finally" ":" \token{suite}]} \production{try2_stmt} {"try" ":" \token{suite}} \productioncont{"finally" ":" \token{suite}}\end{productionlist} \versionchanged[In previous versions of Python,\keyword{try}...\keyword{except}...\keyword{finally} did not work.\keyword{try}...\keyword{except} had to be nested in\keyword{try}...\keyword{finally}]{2.5} The \keyword{except} clause(s) specify one or more exception handlers.When no exception occurs in the\keyword{try} clause, no exception handler is executed. When anexception occurs in the \keyword{try} suite, a search for an exceptionhandler is started. This search inspects the except clauses in turn untilone is found that matches the exception. An expression-less exceptclause, if present, must be last; it matches any exception. For anexcept clause with an expression, that expression is evaluated, and theclause matches the exception if the resulting object is ``compatible''with the exception. An object is compatible with an exception if itis the class or a base class of the exception object, a tuplecontaining an item compatible with the exception, or, in the(deprecated) case of string exceptions, is the raised string itself(note that the object identities must match, i.e. it must be the samestring object, not just a string with the same value).\kwindex{except} If no except clause matches the exception, the search for an exceptionhandler continues in the surrounding code and on the invocation stack.\footnote{The exception is propogated to the invocation stack only ifthere is no \keyword{finally} clause that negates the exception.} If the evaluation of an expression in the header of an except clauseraises an exception, the original search for a handler is canceledand a search starts for the new exception in the surrounding code andon the call stack (it is treated as if the entire \keyword{try} statementraised the exception). When a matching except clause is found, the exception is assigned tothe target specified in that except clause, if present, and the exceptclause's suite is executed. All except clauses must have anexecutable block. When the end of this block is reached, executioncontinues normally after the entire try statement. (This means thatif two nested handlers exist for the same exception, and the exceptionoccurs in the try clause of the inner handler, the outer handler willnot handle the exception.) Before an except clause's suite is executed, details about theexception are assigned to three variables in the\module{sys}\refbimodindex{sys} module: \code{sys.exc_type} receivesthe object identifying the exception; \code{sys.exc_value} receivesthe exception's parameter; \code{sys.exc_traceback} receives atraceback object\obindex{traceback} (see section~\ref{traceback})identifying the point in the program where the exception occurred.These details are also available through the \function{sys.exc_info()}function, which returns a tuple \code{(\var{exc_type}, \var{exc_value},\var{exc_traceback})}. Use of the corresponding variables isdeprecated in favor of this function, since their use is unsafe in athreaded program. As of Python 1.5, the variables are restored totheir previous values (before the call) when returning from a functionthat handled an exception.\withsubitem{(in module sys)}{\ttindex{exc_type} \ttindex{exc_value}\ttindex{exc_traceback}} The optional \keyword{else} clause is executed if and when controlflows off the end of the \keyword{try} clause.\footnote{ Currently, control ``flows off the end'' except in the case of an exception or the execution of a \keyword{return}, \keyword{continue}, or \keyword{break} statement.} Exceptions in the \keyword{else} clause are not handled by thepreceding \keyword{except} clauses.\kwindex{else}\stindex{return}\stindex{break}\stindex{continue} If \keyword{finally} is present, it specifies a `cleanup' handler. The\keyword{try} clause is executed, including any \keyword{except} and\keyword{else} clauses. If an exception occurs in any of the clausesand is not handled, the exception is temporarily saved. The\keyword{finally} clause is executed. If there is a saved exception,it is re-raised at the end of the \keyword{finally} clause.If the \keyword{finally} clause raises another exception orexecutes a \keyword{return} or \keyword{break} statement, the savedexception is lost. The exception information is not available to theprogram during execution of the \keyword{finally} clause.\kwindex{finally} When a \keyword{return}, \keyword{break} or \keyword{continue} statement isexecuted in the \keyword{try} suite of a \keyword{try}...\keyword{finally}statement, the \keyword{finally} clause is also executed `on the way out.' A\keyword{continue} statement is illegal in the \keyword{finally} clause.(The reason is a problem with the current implementation --- thisrestriction may be lifted in the future).\stindex{return}\stindex{break}\stindex{continue} Additional information on exceptions can be found insection~\ref{exceptions}, and information on using the \keyword{raise}statement to generate exceptions may be found in section~\ref{raise}. \section{The \keyword{with} statement\label{with}}\stindex{with} \versionadded{2.5} The \keyword{with} statement is used to wrap the execution of a blockwith methods defined by a context manager (seesection~\ref{context-managers}). This allows common\keyword{try}...\keyword{except}...\keyword{finally} usage patterns tobe encapsulated for convenient reuse. \begin{productionlist} \production{with_stmt} {"with" \token{expression} ["as" \token{target}] ":" \token{suite}}\end{productionlist} The execution of the \keyword{with} statement proceeds as follows: \begin{enumerate} \item The context expression is evaluated to obtain a context manager. \item The context manager's \method{__enter__()} method is invoked. \item If a target was included in the \keyword{with}statement, the return value from \method{__enter__()} is assigned to it. \note{The \keyword{with} statement guarantees that if the\method{__enter__()} method returns without an error, then\method{__exit__()} will always be called. Thus, if an error occursduring the assignment to the target list, it will be treated the same asan error occurring within the suite would be. See step 5 below.} \item The suite is executed. \item The context manager's \method{__exit__()} method is invoked. Ifan exception caused the suite to be exited, its type, value, andtraceback are passed as arguments to \method{__exit__()}. Otherwise,three \constant{None} arguments are supplied. If the suite was exited due to an exception, and the returnvalue from the \method{__exit__()} method was false, the exception isreraised. If the return value was true, the exception is suppressed, andexecution continues with the statement following the \keyword{with}statement. If the suite was exited for any reason other than an exception, thereturn value from \method{__exit__()} is ignored, and execution proceedsat the normal location for the kind of exit that was taken. \end{enumerate} \begin{notice}In Python 2.5, the \keyword{with} statement is only allowedwhen the \code{with_statement} feature has been enabled. It will alwaysbe enabled in Python 2.6. This \code{__future__} import statement canbe used to enable the feature: \begin{verbatim}from __future__ import with_statement\end{verbatim}\end{notice} \begin{seealso} \seepep{0343}{The "with" statement} {The specification, background, and examples for the Python \keyword{with} statement.}\end{seealso} \section{Function definitions\label{function}}\indexii{function}{definition}\stindex{def} A function definition defines a user-defined function object (seesection~\ref{types}):\obindex{user-defined function}\obindex{function} \begin{productionlist} \production{funcdef} {[\token{decorators}] "def" \token{funcname} "(" [\token{parameter_list}] ")" ":" \token{suite}} \production{decorators} {\token{decorator}+} \production{decorator} {"@" \token{dotted_name} ["(" [\token{argument_list} [","]] ")"] NEWLINE} \production{dotted_name} {\token{identifier} ("." \token{identifier})*} \production{parameter_list} {(\token{defparameter} ",")*} \productioncont{(~~"*" \token{identifier} [, "**" \token{identifier}]} \productioncont{ | "**" \token{identifier}} \productioncont{ | \token{defparameter} [","] )} \production{defparameter} {\token{parameter} ["=" \token{expression}]} \production{sublist} {\token{parameter} ("," \token{parameter})* [","]} \production{parameter} {\token{identifier} | "(" \token{sublist} ")"} \production{funcname} {\token{identifier}}\end{productionlist} A function definition is an executable statement. Its execution bindsthe function name in the current local namespace to a function object(a wrapper around the executable code for the function). Thisfunction object contains a reference to the current global namespaceas the global namespace to be used when the function is called.\indexii{function}{name}\indexii{name}{binding} The function definition does not execute the function body; this getsexecuted only when the function is called. A function definition may be wrapped by one or more decorator expressions.Decorator expressions are evaluated when the function is defined, in the scopethat contains the function definition. The result must be a callable,which is invoked with the function object as the only argument.The returned value is bound to the function name instead of the functionobject. Multiple decorators are applied in nested fashion.For example, the following code: \begin{verbatim}@f1(arg)@f2def func(): pass\end{verbatim} is equivalent to: \begin{verbatim}def func(): passfunc = f1(arg)(f2(func))\end{verbatim} When one or more top-level parameters have the form \var{parameter}\code{=} \var{expression}, the function is said to have ``defaultparameter values.'' For a parameter with adefault value, the corresponding argument may be omitted from a call,in which case the parameter's default value is substituted. If aparameter has a default value, all following parameters must also havea default value --- this is a syntactic restriction that is notexpressed by the grammar.\indexiii{default}{parameter}{value} \strong{Default parameter values are evaluated when the functiondefinition is executed.} This means that the expression is evaluatedonce, when the function is defined, and that that same``pre-computed'' value is used for each call. This is especiallyimportant to understand when a default parameter is a mutable object,such as a list or a dictionary: if the function modifies the object(e.g. by appending an item to a list), the default value is in effectmodified. This is generally not what was intended. A way around this is to use \code{None} as the default, and explicitly test for it inthe body of the function, e.g.: \begin{verbatim}def whats_on_the_telly(penguin=None): if penguin is None: penguin = [] penguin.append("property of the zoo") return penguin\end{verbatim} Function call semantics are described in more detail insection~\ref{calls}.A function call always assigns values to all parameters mentioned inthe parameter list, either from position arguments, from keywordarguments, or from default values. If the form ``\code{*identifier}''is present, it is initialized to a tuple receiving any excesspositional parameters, defaulting to the empty tuple. If the form``\code{**identifier}'' is present, it is initialized to a newdictionary receiving any excess keyword arguments, defaulting to anew empty dictionary. It is also possible to create anonymous functions (functions not boundto a name), for immediate use in expressions. This uses lambda forms,described in section~\ref{lambda}. Note that the lambda form ismerely a shorthand for a simplified function definition; a functiondefined in a ``\keyword{def}'' statement can be passed around orassigned to another name just like a function defined by a lambdaform. The ``\keyword{def}'' form is actually more powerful since itallows the execution of multiple statements.\indexii{lambda}{form} \strong{Programmer's note:} Functions are first-class objects. A``\code{def}'' form executed inside a function definition defines alocal function that can be returned or passed around. Free variablesused in the nested function can access the local variables of thefunction containing the def. See section~\ref{naming} for details. Lines 1–500 of 544. Load more
\chapter{Compound statements\label{compound}} \indexii{compound}{statement} Compound statements contain (groups of) other statements; they affect or control the execution of those other statements in some way. In general, compound statements span multiple lines, although in simple incarnations a whole compound statement may be contained in one line. The \keyword{if}, \keyword{while} and \keyword{for} statements implement traditional control flow constructs. \keyword{try} specifies exception handlers and/or cleanup code for a group of statements. Function and class definitions are also syntactically compound statements. Compound statements consist of one or more `clauses.' A clause consists of a header and a `suite.' The clause headers of a particular compound statement are all at the same indentation level. Each clause header begins with a uniquely identifying keyword and ends with a colon. A suite is a group of statements controlled by a clause. A suite can be one or more semicolon-separated simple statements on the same line as the header, following the header's colon, or it can be one or more indented statements on subsequent lines. Only the latter form of suite can contain nested compound statements; the following is illegal, mostly because it wouldn't be clear to which \keyword{if} clause a following \keyword{else} clause would belong: \index{clause} \index{suite} \begin{verbatim} if test1: if test2: print x \end{verbatim} Also note that the semicolon binds tighter than the colon in this context, so that in the following example, either all or none of the \keyword{print} statements are executed: \begin{verbatim} if x < y < z: print x; print y; print z \end{verbatim} Summarizing: \begin{productionlist} \production{compound_stmt} {\token{if_stmt}} \productioncont{| \token{while_stmt}} \productioncont{| \token{for_stmt}} \productioncont{| \token{try_stmt}} \productioncont{| \token{with_stmt}} \productioncont{| \token{funcdef}} \productioncont{| \token{classdef}} \production{suite} {\token{stmt_list} NEWLINE | NEWLINE INDENT \token{statement}+ DEDENT} \production{statement} {\token{stmt_list} NEWLINE | \token{compound_stmt}} \production{stmt_list} {\token{simple_stmt} (";" \token{simple_stmt})* [";"]} \end{productionlist} Note that statements always end in a \code{NEWLINE}\index{NEWLINE token} possibly followed by a \code{DEDENT}.\index{DEDENT token} Also note that optional continuation clauses always begin with a keyword that cannot start a statement, thus there are no ambiguities (the `dangling \keyword{else}' problem is solved in Python by requiring nested \keyword{if} statements to be indented). \indexii{dangling}{else} The formatting of the grammar rules in the following sections places each clause on a separate line for clarity. \section{The \keyword{if} statement\label{if}} \stindex{if} The \keyword{if} statement is used for conditional execution: \begin{productionlist} \production{if_stmt} {"if" \token{expression} ":" \token{suite}} \productioncont{( "elif" \token{expression} ":" \token{suite} )*} \productioncont{["else" ":" \token{suite}]} \end{productionlist} It selects exactly one of the suites by evaluating the expressions one by one until one is found to be true (see section~\ref{Booleans} for the definition of true and false); then that suite is executed (and no other part of the \keyword{if} statement is executed or evaluated). If all expressions are false, the suite of the \keyword{else} clause, if present, is executed. \kwindex{elif} \kwindex{else} \section{The \keyword{while} statement\label{while}} \stindex{while} \indexii{loop}{statement} The \keyword{while} statement is used for repeated execution as long as an expression is true: \begin{productionlist} \production{while_stmt} {"while" \token{expression} ":" \token{suite}} \productioncont{["else" ":" \token{suite}]} \end{productionlist} This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the \keyword{else} clause, if present, is executed and the loop terminates. \kwindex{else} A \keyword{break} statement executed in the first suite terminates the loop without executing the \keyword{else} clause's suite. A \keyword{continue} statement executed in the first suite skips the rest of the suite and goes back to testing the expression. \stindex{break} \stindex{continue} \section{The \keyword{for} statement\label{for}} \stindex{for} \indexii{loop}{statement} The \keyword{for} statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object: \obindex{sequence} \begin{productionlist} \production{for_stmt} {"for" \token{target_list} "in" \token{expression_list} ":" \token{suite}} \productioncont{["else" ":" \token{suite}]} \end{productionlist} The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the {}\code{expression_list}. The suite is then executed once for each item provided by the iterator, in the order of ascending indices. Each item in turn is assigned to the target list using the standard rules for assignments, and then the suite is executed. When the items are exhausted (which is immediately when the sequence is empty), the suite in the \keyword{else} clause, if present, is executed, and the loop terminates. \kwindex{in} \kwindex{else} \indexii{target}{list} A \keyword{break} statement executed in the first suite terminates the loop without executing the \keyword{else} clause's suite. A \keyword{continue} statement executed in the first suite skips the rest of the suite and continues with the next item, or with the \keyword{else} clause if there was no next item. \stindex{break} \stindex{continue} The suite may assign to the variable(s) in the target list; this does not affect the next item assigned to it. The target list is not deleted when the loop is finished, but if the sequence is empty, it will not have been assigned to at all by the loop. Hint: the built-in function \function{range()} returns a sequence of integers suitable to emulate the effect of Pascal's \code{for i := a to b do}; e.g., \code{range(3)} returns the list \code{[0, 1, 2]}. \bifuncindex{range} \indexii{Pascal}{language} \warning{There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, i.e. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g., \index{loop!over mutable sequence} \index{mutable sequence!loop over}} \begin{verbatim} for x in a[:]: if x < 0: a.remove(x) \end{verbatim} \section{The \keyword{try} statement\label{try}} \stindex{try} The \keyword{try} statement specifies exception handlers and/or cleanup code for a group of statements: \begin{productionlist} \production{try_stmt} {try1_stmt | try2_stmt} \production{try1_stmt} {"try" ":" \token{suite}} \productioncont{("except" [\token{expression} ["," \token{target}]] ":" \token{suite})+} \productioncont{["else" ":" \token{suite}]} \productioncont{["finally" ":" \token{suite}]} \production{try2_stmt} {"try" ":" \token{suite}} \productioncont{"finally" ":" \token{suite}} \end{productionlist} \versionchanged[In previous versions of Python, \keyword{try}...\keyword{except}...\keyword{finally} did not work. \keyword{try}...\keyword{except} had to be nested in \keyword{try}...\keyword{finally}]{2.5} The \keyword{except} clause(s) specify one or more exception handlers. When no exception occurs in the \keyword{try} clause, no exception handler is executed. When an exception occurs in the \keyword{try} suite, a search for an exception handler is started. This search inspects the except clauses in turn until one is found that matches the exception. An expression-less except clause, if present, must be last; it matches any exception. For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is ``compatible'' with the exception. An object is compatible with an exception if it is the class or a base class of the exception object, a tuple containing an item compatible with the exception, or, in the (deprecated) case of string exceptions, is the raised string itself (note that the object identities must match, i.e. it must be the same string object, not just a string with the same value). \kwindex{except} If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. \footnote{The exception is propogated to the invocation stack only if there is no \keyword{finally} clause that negates the exception.} If the evaluation of an expression in the header of an except clause raises an exception, the original search for a handler is canceled and a search starts for the new exception in the surrounding code and on the call stack (it is treated as if the entire \keyword{try} statement raised the exception). When a matching except clause is found, the exception is assigned to the target specified in that except clause, if present, and the except clause's suite is executed. All except clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.) Before an except clause's suite is executed, details about the exception are assigned to three variables in the \module{sys}\refbimodindex{sys} module: \code{sys.exc_type} receives the object identifying the exception; \code{sys.exc_value} receives the exception's parameter; \code{sys.exc_traceback} receives a traceback object\obindex{traceback} (see section~\ref{traceback}) identifying the point in the program where the exception occurred. These details are also available through the \function{sys.exc_info()} function, which returns a tuple \code{(\var{exc_type}, \var{exc_value}, \var{exc_traceback})}. Use of the corresponding variables is deprecated in favor of this function, since their use is unsafe in a threaded program. As of Python 1.5, the variables are restored to their previous values (before the call) when returning from a function that handled an exception. \withsubitem{(in module sys)}{\ttindex{exc_type} \ttindex{exc_value}\ttindex{exc_traceback}} The optional \keyword{else} clause is executed if and when control flows off the end of the \keyword{try} clause.\footnote{ Currently, control ``flows off the end'' except in the case of an exception or the execution of a \keyword{return}, \keyword{continue}, or \keyword{break} statement. } Exceptions in the \keyword{else} clause are not handled by the preceding \keyword{except} clauses. \kwindex{else} \stindex{return} \stindex{break} \stindex{continue} If \keyword{finally} is present, it specifies a `cleanup' handler. The \keyword{try} clause is executed, including any \keyword{except} and \keyword{else} clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The \keyword{finally} clause is executed. If there is a saved exception, it is re-raised at the end of the \keyword{finally} clause. If the \keyword{finally} clause raises another exception or executes a \keyword{return} or \keyword{break} statement, the saved exception is lost. The exception information is not available to the program during execution of the \keyword{finally} clause. \kwindex{finally} When a \keyword{return}, \keyword{break} or \keyword{continue} statement is executed in the \keyword{try} suite of a \keyword{try}...\keyword{finally} statement, the \keyword{finally} clause is also executed `on the way out.' A \keyword{continue} statement is illegal in the \keyword{finally} clause. (The reason is a problem with the current implementation --- this restriction may be lifted in the future). \stindex{return} \stindex{break} \stindex{continue} Additional information on exceptions can be found in section~\ref{exceptions}, and information on using the \keyword{raise} statement to generate exceptions may be found in section~\ref{raise}. \section{The \keyword{with} statement\label{with}} \stindex{with} \versionadded{2.5} The \keyword{with} statement is used to wrap the execution of a block with methods defined by a context manager (see section~\ref{context-managers}). This allows common \keyword{try}...\keyword{except}...\keyword{finally} usage patterns to be encapsulated for convenient reuse. \begin{productionlist} \production{with_stmt} {"with" \token{expression} ["as" \token{target}] ":" \token{suite}} \end{productionlist} The execution of the \keyword{with} statement proceeds as follows: \begin{enumerate} \item The context expression is evaluated to obtain a context manager. \item The context manager's \method{__enter__()} method is invoked. \item If a target was included in the \keyword{with} statement, the return value from \method{__enter__()} is assigned to it. \note{The \keyword{with} statement guarantees that if the \method{__enter__()} method returns without an error, then \method{__exit__()} will always be called. Thus, if an error occurs during the assignment to the target list, it will be treated the same as an error occurring within the suite would be. See step 5 below.} \item The suite is executed. \item The context manager's \method{__exit__()} method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to \method{__exit__()}. Otherwise, three \constant{None} arguments are supplied. If the suite was exited due to an exception, and the return value from the \method{__exit__()} method was false, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the \keyword{with} statement. If the suite was exited for any reason other than an exception, the return value from \method{__exit__()} is ignored, and execution proceeds at the normal location for the kind of exit that was taken. \end{enumerate} \begin{notice} In Python 2.5, the \keyword{with} statement is only allowed when the \code{with_statement} feature has been enabled. It will always be enabled in Python 2.6. This \code{__future__} import statement can be used to enable the feature: \begin{verbatim} from __future__ import with_statement \end{verbatim} \end{notice} \begin{seealso} \seepep{0343}{The "with" statement} {The specification, background, and examples for the Python \keyword{with} statement.} \end{seealso} \section{Function definitions\label{function}} \indexii{function}{definition} \stindex{def} A function definition defines a user-defined function object (see section~\ref{types}): \obindex{user-defined function} \obindex{function} \begin{productionlist} \production{funcdef} {[\token{decorators}] "def" \token{funcname} "(" [\token{parameter_list}] ")" ":" \token{suite}} \production{decorators} {\token{decorator}+} \production{decorator} {"@" \token{dotted_name} ["(" [\token{argument_list} [","]] ")"] NEWLINE} \production{dotted_name} {\token{identifier} ("." \token{identifier})*} \production{parameter_list} {(\token{defparameter} ",")*} \productioncont{(~~"*" \token{identifier} [, "**" \token{identifier}]} \productioncont{ | "**" \token{identifier}} \productioncont{ | \token{defparameter} [","] )} \production{defparameter} {\token{parameter} ["=" \token{expression}]} \production{sublist} {\token{parameter} ("," \token{parameter})* [","]} \production{parameter} {\token{identifier} | "(" \token{sublist} ")"} \production{funcname} {\token{identifier}} \end{productionlist} A function definition is an executable statement. Its execution binds the function name in the current local namespace to a function object (a wrapper around the executable code for the function). This function object contains a reference to the current global namespace as the global namespace to be used when the function is called. \indexii{function}{name} \indexii{name}{binding} The function definition does not execute the function body; this gets executed only when the function is called. A function definition may be wrapped by one or more decorator expressions. Decorator expressions are evaluated when the function is defined, in the scope that contains the function definition. The result must be a callable, which is invoked with the function object as the only argument. The returned value is bound to the function name instead of the function object. Multiple decorators are applied in nested fashion. For example, the following code: \begin{verbatim} @f1(arg) @f2 def func(): pass \end{verbatim} is equivalent to: \begin{verbatim} def func(): pass func = f1(arg)(f2(func)) \end{verbatim} When one or more top-level parameters have the form \var{parameter} \code{=} \var{expression}, the function is said to have ``default parameter values.'' For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter's default value is substituted. If a parameter has a default value, all following parameters must also have a default value --- this is a syntactic restriction that is not expressed by the grammar. \indexiii{default}{parameter}{value} \strong{Default parameter values are evaluated when the function definition is executed.} This means that the expression is evaluated once, when the function is defined, and that that same ``pre-computed'' value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use \code{None} as the default, and explicitly test for it in the body of the function, e.g.: \begin{verbatim} def whats_on_the_telly(penguin=None): if penguin is None: penguin = [] penguin.append("property of the zoo") return penguin \end{verbatim} Function call semantics are described in more detail in section~\ref{calls}. A function call always assigns values to all parameters mentioned in the parameter list, either from position arguments, from keyword arguments, or from default values. If the form ``\code{*identifier}'' is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form ``\code{**identifier}'' is present, it is initialized to a new dictionary receiving any excess keyword arguments, defaulting to a new empty dictionary. It is also possible to create anonymous functions (functions not bound to a name), for immediate use in expressions. This uses lambda forms, described in section~\ref{lambda}. Note that the lambda form is merely a shorthand for a simplified function definition; a function defined in a ``\keyword{def}'' statement can be passed around or assigned to another name just like a function defined by a lambda form. The ``\keyword{def}'' form is actually more powerful since it allows the execution of multiple statements. \indexii{lambda}{form} \strong{Programmer's note:} Functions are first-class objects. A ``\code{def}'' form executed inside a function definition defines a local function that can be returned or passed around. Free variables used in the nested function can access the local variables of the function containing the def. See section~\ref{naming} for details.