\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{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 a sequence. The

suite is then executed once for each item in the sequence, 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}

{\token{try_exc_stmt} | \token{try_fin_stmt}}

\production{try_exc_stmt}

{"try" ":" \token{suite}}

\productioncont{("except" [\token{expression}

["," \token{target}]] ":" \token{suite})+}

\productioncont{["else" ":" \token{suite}]}

\production{try_fin_stmt}

{"try" ":" \token{suite}

"finally" ":" \token{suite}}

\end{productionlist}

There are two forms of \keyword{try} statement:

\keyword{try}...\keyword{except} and

\keyword{try}...\keyword{finally}. These forms cannot be mixed (but

they can be nested in each other).

The \keyword{try}...\keyword{except} form specifies one or more

exception handlers

(the \keyword{except} clauses). 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 either the object that identifies the exception, or (for exceptions

that are classes) it is a base class of the exception, or it is a

tuple containing an item that is compatible with the exception. Note

that the object identities must match, i.e. it must be the same

object, not just an object 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.

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's parameter 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}

The \keyword{try}...\keyword{finally} form specifies a `cleanup' handler. The

\keyword{try} clause is executed. When no exception occurs, the

\keyword{finally} clause is executed. When an exception occurs in the

\keyword{try} clause, the exception is temporarily saved, the

\keyword{finally} clause is executed, and then the saved exception is

re-raised. If the \keyword{finally} clause raises another exception or

executes a \keyword{return} or \keyword{break} statement, the saved

exception is lost. 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). 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}

\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}

{"def" \token{funcname} "(" [\token{parameter_list}] ")"

":" \token{suite}}

\production{parameter_list}

{(\token{defparameter} ",")*}

\productioncont{("*" \token{identifier} [, "**" \token{identifier}]}

\productioncont{| "**" \token{identifier}

| \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.

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.

\section{Class definitions\label{class}}

\indexii{class}{definition}

\stindex{class}

A class definition defines a class object (see section \ref{types}):

\obindex{class}

\begin{productionlist}

\production{classdef}

{"class" \token{classname} [\token{inheritance}] ":"

\token{suite}}

\production{inheritance}

{"(" [\token{expression_list}] ")"}

\production{classname}

{\token{identifier}}

\end{productionlist}

A class definition is an executable statement. It first evaluates the

inheritance list, if present. Each item in the inheritance list

should evaluate to a class object. The class's suite is then executed

in a new execution frame (see section \ref{naming}), using a newly

created local namespace and the original global namespace.

(Usually, the suite contains only function definitions.) When the

class's suite finishes execution, its execution frame is discarded but

its local namespace is saved. A class object is then created using

the inheritance list for the base classes and the saved local

namespace for the attribute dictionary. The class name is bound to this

class object in the original local namespace.

\index{inheritance}

\indexii{class}{name}

\indexii{name}{binding}

\indexii{execution}{frame}

\strong{Programmer's note:} variables defined in the class definition

are class variables; they are shared by all instances. To define

instance variables, they must be given a value in the the

\method{__init__()} method or in another method. Both class and

instance variables are accessible through the notation

``\code{self.name}'', and an instance variable hides a class variable

with the same name when accessed in this way. Class variables with

immutable values can be used as defaults for instance variables.