\chapter{Simple statements \label{simple}}

\indexii{simple}{statement}

Simple statements are comprised within a single logical line.

Several simple statements may occur on a single line separated

by semicolons. The syntax for simple statements is:

\begin{verbatim}

simple_stmt: expression_stmt

| assert_stmt

| assignment_stmt

| augmented_assignment_stmt

| pass_stmt

| del_stmt

| print_stmt

| return_stmt

| raise_stmt

| break_stmt

| continue_stmt

| import_stmt

| global_stmt

| exec_stmt

\end{verbatim}

\section{Expression statements \label{exprstmts}}

\indexii{expression}{statement}

Expression statements are used (mostly interactively) to compute and

write a value, or (usually) to call a procedure (a function that

returns no meaningful result; in Python, procedures return the value

\code{None}). Other uses of expression statements are allowed and

occasionally useful. The syntax for an expression statement is:

\begin{verbatim}

expression_stmt: expression_list

\end{verbatim}

An expression statement evaluates the expression list (which may be a

single expression).

\indexii{expression}{list}

In interactive mode, if the value is not \code{None}, it is converted

to a string using the built-in \function{repr()}\bifuncindex{repr}

function and the resulting string is written to standard output (see

section \ref{print}) on a line by itself. (Expression statements

yielding None are not written, so that procedure calls do not cause

any output.)

\ttindex{None}

\indexii{string}{conversion}

\index{output}

\indexii{standard}{output}

\indexii{writing}{values}

\indexii{procedure}{call}

\section{Assert statements \label{assert}}

Assert statements\stindex{assert} are a convenient way to insert

debugging assertions\indexii{debugging}{assertions} into a program:

\begin{verbatim}

assert_statement: "assert" expression ["," expression]

\end{verbatim}

The simple form, \samp{assert expression}, is equivalent to

\begin{verbatim}

if __debug__:

if not expression: raise AssertionError

\end{verbatim}

The extended form, \samp{assert expression1, expression2}, is

equivalent to

\begin{verbatim}

if __debug__:

if not expression1: raise AssertionError, expression2

\end{verbatim}

These equivalences assume that \code{__debug__}\ttindex{__debug__} and

\exception{AssertionError}\exindex{AssertionError} refer to the built-in

variables with those names. In the current implementation, the

built-in variable \code{__debug__} is 1 under normal circumstances, 0

when optimization is requested (command line option -O). The current

code generator emits no code for an assert statement when optimization

is requested at compile time. Note that it is unnecessary to include

the source code for the expression that failed in the error message;

it will be displayed as part of the stack trace.

\section{Assignment statements \label{assignment}}

Assignment statements\indexii{assignment}{statement} are used to

(re)bind names to values and to modify attributes or items of mutable

objects:

\indexii{binding}{name}

\indexii{rebinding}{name}

\obindex{mutable}

\indexii{attribute}{assignment}

\begin{verbatim}

assignment_stmt: (target_list "=")+ expression_list

target_list: target ("," target)* [","]

target: identifier | "(" target_list ")" | "[" target_list "]"

| attributeref | subscription | slicing

\end{verbatim}

(See section \ref{primaries} for the syntax definitions for the last

three symbols.)

An assignment statement evaluates the expression list (remember that

this can be a single expression or a comma-separated list, the latter

yielding a tuple) and assigns the single resulting object to each of

the target lists, from left to right.

\indexii{expression}{list}

Assignment is defined recursively depending on the form of the target

(list). When a target is part of a mutable object (an attribute

reference, subscription or slicing), the mutable object must

ultimately perform the assignment and decide about its validity, and

may raise an exception if the assignment is unacceptable. The rules

observed by various types and the exceptions raised are given with the

definition of the object types (see section \ref{types}).

\index{target}

\indexii{target}{list}

Assignment of an object to a target list is recursively defined as

follows.

\indexiii{target}{list}{assignment}

\begin{itemize}

\item

If the target list is a single target: The object is assigned to that

target.

\item

If the target list is a comma-separated list of targets: The object

must be a sequence with the same number of items as the there are

targets in the target list, and the items are assigned, from left to

right, to the corresponding targets. (This rule is relaxed as of

Python 1.5; in earlier versions, the object had to be a tuple. Since

strings are sequences, an assignment like \samp{a, b = "xy"} is

now legal as long as the string has the right length.)

\end{itemize}

Assignment of an object to a single target is recursively defined as

follows.

\begin{itemize} % nested

\item

If the target is an identifier (name):

\begin{itemize}

\item

If the name does not occur in a \keyword{global} statement in the current

code block: the name is bound to the object in the current local

namespace.

\stindex{global}

\item

Otherwise: the name is bound to the object in the current global

namespace.

\end{itemize} % nested

The name is rebound if it was already bound. This may cause the

reference count for the object previously bound to the name to reach

zero, causing the object to be deallocated and its

destructor\index{destructor} (if it has one) to be called.

\item

If the target is a target list enclosed in parentheses or in square

brackets: The object must be a sequence with the same number of items

as there are targets in the target list, and its items are assigned,

from left to right, to the corresponding targets.

\item

If the target is an attribute reference: The primary expression in the

reference is evaluated. It should yield an object with assignable

attributes; if this is not the case, \exception{TypeError} is raised. That

object is then asked to assign the assigned object to the given

attribute; if it cannot perform the assignment, it raises an exception

(usually but not necessarily \exception{AttributeError}).

\indexii{attribute}{assignment}

\item

If the target is a subscription: The primary expression in the

reference is evaluated. It should yield either a mutable sequence

object (e.g., a list) or a mapping object (e.g., a dictionary). Next,

the subscript expression is evaluated.

\indexii{subscription}{assignment}

\obindex{mutable}

If the primary is a mutable sequence object (e.g., a list), the subscript

must yield a plain integer. If it is negative, the sequence's length

is added to it. The resulting value must be a nonnegative integer

less than the sequence's length, and the sequence is asked to assign

the assigned object to its item with that index. If the index is out

of range, \exception{IndexError} is raised (assignment to a subscripted

sequence cannot add new items to a list).

\obindex{sequence}

\obindex{list}

If the primary is a mapping object (e.g., a dictionary), the subscript must

have a type compatible with the mapping's key type, and the mapping is

then asked to create a key/datum pair which maps the subscript to

the assigned object. This can either replace an existing key/value

pair with the same key value, or insert a new key/value pair (if no

key with the same value existed).

\obindex{mapping}

\obindex{dictionary}

\item

If the target is a slicing: The primary expression in the reference is

evaluated. It should yield a mutable sequence object (e.g., a list). The

assigned object should be a sequence object of the same type. Next,

the lower and upper bound expressions are evaluated, insofar they are

present; defaults are zero and the sequence's length. The bounds

should evaluate to (small) integers. If either bound is negative, the

sequence's length is added to it. The resulting bounds are clipped to

lie between zero and the sequence's length, inclusive. Finally, the

sequence object is asked to replace the slice with the items of the

assigned sequence. The length of the slice may be different from the

length of the assigned sequence, thus changing the length of the

target sequence, if the object allows it.

\indexii{slicing}{assignment}

\end{itemize}

(In the current implementation, the syntax for targets is taken

to be the same as for expressions, and invalid syntax is rejected

during the code generation phase, causing less detailed error

messages.)

WARNING: Although the definition of assignment implies that overlaps

between the left-hand side and the right-hand side are `safe' (e.g.,

\samp{a, b = b, a} swaps two variables), overlaps \emph{within} the

collection of assigned-to variables are not safe! For instance, the

following program prints \samp{[0, 2]}:

\begin{verbatim}

x = [0, 1]

i = 0

i, x[i] = 1, 2

print x

\end{verbatim}

\subsection{Augmented Assignment statements \label{augassign}}

Augmented assignment is the combination, in a single statement, of a binary

operation and an assignment statement:

\indexii{augmented}{assignment}

\index{statement!assignment, augmented}

\begin{verbatim}

augmented_assignment_stmt: target augop expression_list

augop: "+=" | "-=" | "*=" | "/=" | "%=" | "**="

| ">>=" | "<<=" | "&=" | "^=" | "|="

target: identifier | "(" target_list ")" | "[" target_list "]"

| attributeref | subscription | slicing

\end{verbatim}

(See section \ref{primaries} for the syntax definitions for the last

three symbols.)

An augmented assignment evaluates the target (which, unlike with normal

assignment statements, cannot be a tuple) and the expression list, performs

the binary operation specific to the type of assignment on the two operands,

and assigns the result to the original target. The target is only evaluated

once.

An augmented assignment expression like \code{x += 1} can be rewritten as

\code{x = x + 1} to achieve a similar, but not exactly equal effect. In the

augmented version, \code{x} is only evaluated once. Also, when possible, the

actual operation is performed \emph{in-place}, meaning that rather than

creating a new object and assigning that to the target, the old object is

modified instead.

With the exception of assigning to tuples and multiple targets in a single

statement, the assignment done by augmented assignment statements is handled

the same way as normal assignments. Similarly, with the exception of the

possible \emph{in-place} behaviour, the binary operation performed by

augmented assignment is the same as the normal binary operations.

\section{The \keyword{pass} statement \label{pass}}

\stindex{pass}

\begin{verbatim}

pass_stmt: "pass"

\end{verbatim}

\keyword{pass} is a null operation --- when it is executed, nothing

happens. It is useful as a placeholder when a statement is

required syntactically, but no code needs to be executed, for example:

\indexii{null}{operation}

\begin{verbatim}

def f(arg): pass # a function that does nothing (yet)

class C: pass # a class with no methods (yet)

\end{verbatim}

\section{The \keyword{del} statement \label{del}}

\stindex{del}

\begin{verbatim}

del_stmt: "del" target_list

\end{verbatim}

Deletion is recursively defined very similar to the way assignment is

defined. Rather that spelling it out in full details, here are some

hints.

\indexii{deletion}{target}

\indexiii{deletion}{target}{list}

Deletion of a target list recursively deletes each target, from left

to right.

Deletion of a name removes the binding of that name (which must exist)

from the local or global namespace, depending on whether the name

occurs in a \keyword{global} statement in the same code block.

\stindex{global}

\indexii{unbinding}{name}

Deletion of attribute references, subscriptions and slicings

is passed to the primary object involved; deletion of a slicing

is in general equivalent to assignment of an empty slice of the

right type (but even this is determined by the sliced object).

\indexii{attribute}{deletion}

\section{The \keyword{print} statement \label{print}}

\stindex{print}

\begin{verbatim}

print_stmt: "print" [ expression ("," expression)* [","] ]

\end{verbatim}

\keyword{print} evaluates each expression in turn and writes the

resulting object to standard output (see below). If an object is not

a string, it is first converted to a string using the rules for string

conversions. The (resulting or original) string is then written. A

space is written before each object is (converted and) written, unless

the output system believes it is positioned at the beginning of a

line. This is the case (1) when no characters have yet been written

to standard output, (2) when the last character written to standard

output is \character{\e n}, or (3) when the last write operation on

standard output was not a \keyword{print} statement. (In some cases

it may be functional to write an empty string to standard output for

this reason.)

\index{output}

\indexii{writing}{values}

A \character{\e n} character is written at the end, unless the

\keyword{print} statement ends with a comma. This is the only action

if the statement contains just the keyword \keyword{print}.

\indexii{trailing}{comma}

\indexii{newline}{suppression}

Standard output is defined as the file object named \code{stdout}

in the built-in module \module{sys}. If no such object exists, or if

it does not have a \method{write()} method, a \exception{RuntimeError}

exception is raised.

\indexii{standard}{output}

\refbimodindex{sys}

\withsubitem{(in module sys)}{\ttindex{stdout}}

\exindex{RuntimeError}

\keyword{print} also has an extended form, defined as

\begin{verbatim}

print_stmt: '>>' expression [ (',' expression)+ [','] ] )

\end{verbatim}

In this form, the first expression after the \keyword{>>} must

evaluate to a ``file-like'' object, specifically an object that has a

\method{write()} method as described above. With this extended form,

the subsequent expressions are printed to this file object. If the

first expression evaluates to \code{None}, then \code{sys.stdout} is

used as the file for output.

\section{The \keyword{return} statement \label{return}}

\stindex{return}

\begin{verbatim}

return_stmt: "return" [expression_list]

\end{verbatim}

\keyword{return} may only occur syntactically nested in a function

definition, not within a nested class definition.

\indexii{function}{definition}

\indexii{class}{definition}

If an expression list is present, it is evaluated, else \code{None}

is substituted.

\keyword{return} leaves the current function call with the expression

list (or \code{None}) as return value.

When \keyword{return} passes control out of a \keyword{try} statement

with a \keyword{finally} clause, that \keyword{finally} clause is executed

before really leaving the function.

\kwindex{finally}

\section{The \keyword{raise} statement \label{raise}}

\stindex{raise}

\begin{verbatim}

raise_stmt: "raise" [expression ["," expression ["," expression]]]

\end{verbatim}

If no expressions are present, \keyword{raise} re-raises the last

expression that was raised in the current scope.

Otherwise, \keyword{raise} evaluates its first expression, which must yield

a string, class, or instance object. If there is a second expression,

this is evaluated, else \code{None} is substituted. If the first

expression is a class object, then the second expression may be an

instance of that class or one of its derivatives, and then that

instance is raised. If the second expression is not such an instance,

the given class is instantiated. The argument list for the

instantiation is determined as follows: if the second expression is a

tuple, it is used as the argument list; if it is \code{None}, the

argument list is empty; otherwise, the argument list consists of a

single argument which is the second expression. If the first

expression is an instance object, the second expression must be

\code{None}.

\index{exception}

\indexii{raising}{exception}

If the first object is a string, it then raises the exception

identified by the first object, with the second one (or \code{None})

as its parameter. If the first object is a class or instance,

it raises the exception identified by the class of the instance

determined in the previous step, with the instance as

its parameter.

If a third object is present, and it is not \code{None}, it should be

a traceback object (see section \ref{traceback}), and it is

substituted instead of the current location as the place where the

exception occurred. This is useful to re-raise an exception

transparently in an except clause.

\obindex{traceback}

\section{The \keyword{break} statement \label{break}}

\stindex{break}

\begin{verbatim}

break_stmt: "break"

\end{verbatim}

\keyword{break} may only occur syntactically nested in a \keyword{for}

or \keyword{while} loop, but not nested in a function or class definition

within that loop.

\stindex{for}

\stindex{while}

\indexii{loop}{statement}

It terminates the nearest enclosing loop, skipping the optional

\keyword{else} clause if the loop has one.

\kwindex{else}

If a \keyword{for} loop is terminated by \keyword{break}, the loop control

target keeps its current value.

\indexii{loop control}{target}

When \keyword{break} passes control out of a \keyword{try} statement

with a \keyword{finally} clause, that \keyword{finally} clause is executed

before really leaving the loop.

\kwindex{finally}

\section{The \keyword{continue} statement \label{continue}}

\stindex{continue}

\begin{verbatim}

continue_stmt: "continue"

\end{verbatim}

\keyword{continue} may only occur syntactically nested in a \keyword{for} or

\keyword{while} loop, but not nested in a function or class definition or

\keyword{try} statement within that loop.\footnote{It may

occur within an \keyword{except} or \keyword{else} clause. The

restriction on occurring in the \keyword{try} clause is implementor's

laziness and will eventually be lifted.}

It continues with the next cycle of the nearest enclosing loop.

\stindex{for}

\stindex{while}

\indexii{loop}{statement}

\kwindex{finally}

\section{The \keyword{import} statement \label{import}}

\stindex{import}

\begin{verbatim}

import_stmt: "import" module ["as" name] ("," module ["as" name] )*

| "from" module "import" identifier ["as" name]

("," identifier ["as" name] )*

| "from" module "import" "*"

module: (identifier ".")* identifier

\end{verbatim}

Import statements are executed in two steps: (1) find a module, and

initialize it if necessary; (2) define a name or names in the local

namespace (of the scope where the \keyword{import} statement occurs).

The first form (without \keyword{from}) repeats these steps for each

identifier in the list. The form with \keyword{from} performs step

(1) once, and then performs step (2) repeatedly.

\indexii{importing}{module}

\indexii{name}{binding}

\kwindex{from}

% XXX Need to define what ``initialize'' means here

The system maintains a table of modules that have been initialized,

indexed by module name. This table is

accessible as \code{sys.modules}. When a module name is found in

this table, step (1) is finished. If not, a search for a module

definition is started. When a module is found, it is loaded. Details

of the module searching and loading process are implementation and

platform specific. It generally involves searching for a ``built-in''

module with the given name and then searching a list of locations

given as \code{sys.path}.

\withsubitem{(in module sys)}{\ttindex{modules}}

\ttindex{sys.modules}

\indexii{module}{name}

\indexii{built-in}{module}

\indexii{user-defined}{module}

\refbimodindex{sys}

\indexii{filename}{extension}

\indexiii{module}{search}{path}

If a built-in module is found, its built-in initialization code is

executed and step (1) is finished. If no matching file is found,

\exception{ImportError} is raised. If a file is found, it is parsed,

yielding an executable code block. If a syntax error occurs,

\exception{SyntaxError} is raised. Otherwise, an empty module of the given

name is created and inserted in the module table, and then the code

block is executed in the context of this module. Exceptions during

this execution terminate step (1).

\indexii{module}{initialization}

\exindex{SyntaxError}

\exindex{ImportError}

\index{code block}

When step (1) finishes without raising an exception, step (2) can

begin.

The first form of \keyword{import} statement binds the module name in the

local namespace to the module object, and then goes on to import the

next identifier, if any. If the module name is followed by \keyword{as},

the name following \keyword{as} is used as the local name for the module. To

avoid confusion, you cannot import sub-modules 'as' a different

local name. So 'import module as m' is legal, but 'import module.submod as

s' is not. The latter should be written as 'from module import submod as s',

see below.

The \keyword{from} form does not bind the module name: it goes through the

list of identifiers, looks each one of them up in the module found in step

(1), and binds the name in the local namespace to the object thus found.

Like with the first form of \keyword{import}, an alternate local name can be

supplied by specifying "\keyword{as} localname". If a name is not found,

\exception{ImportError} is raised. If the list of identifiers is replaced

by a star (\samp{*}), all names defined in the module are bound, except

those beginning with an underscore (\character{_}).

\indexii{name}{binding}

\exindex{ImportError}

Names bound by \keyword{import} statements may not occur in

\keyword{global} statements in the same scope.

\stindex{global}

The \keyword{from} form with \samp{*} may only occur in a module scope.

\kwindex{from}

\stindex{from}

(The current implementation does not enforce the latter two

restrictions, but programs should not abuse this freedom, as future

implementations may enforce them or silently change the meaning of the

program.)

\strong{Hierarchical module names:}\indexiii{hierarchical}{module}{names}

when the module names contains one or more dots, the module search

path is carried out differently. The sequence of identifiers up to

the last dot is used to find a ``package''\index{packages}; the final

identifier is then searched inside the package. A package is

generally a subdirectory of a directory on \code{sys.path} that has a

file \file{__init__.py}.\ttindex{__init__.py}

%

[XXX Can't be bothered to spell this out right now; see the URL

\url{http://www.python.org/doc/essays/packages.html} for more details, also

about how the module search works from inside a package.]

[XXX Also should mention __import__().]

\bifuncindex{__import__}

\section{The \keyword{global} statement \label{global}}

\stindex{global}

\begin{verbatim}

global_stmt: "global" identifier ("," identifier)*

\end{verbatim}

The \keyword{global} statement is a declaration which holds for the

entire current code block. It means that the listed identifiers are to be

interpreted as globals. While \emph{using} global names is automatic

if they are not defined in the local scope, \emph{assigning} to global

names would be impossible without \keyword{global}.

\indexiii{global}{name}{binding}

Names listed in a \keyword{global} statement must not be used in the same

code block textually preceding that \keyword{global} statement.

Names listed in a \keyword{global} statement must not be defined as formal

parameters or in a \keyword{for} loop control target, \keyword{class}

definition, function definition, or \keyword{import} statement.

(The current implementation does not enforce the latter two

restrictions, but programs should not abuse this freedom, as future

implementations may enforce them or silently change the meaning of the

program.)

\strong{Programmer's note:}

the \keyword{global} is a directive to the parser. It

applies only to code parsed at the same time as the \keyword{global}

statement. In particular, a \keyword{global} statement contained in an

\keyword{exec} statement does not affect the code block \emph{containing}

the \keyword{exec} statement, and code contained in an \keyword{exec}

statement is unaffected by \keyword{global} statements in the code

containing the \keyword{exec} statement. The same applies to the

\function{eval()}, \function{execfile()} and \function{compile()} functions.

\stindex{exec}

\bifuncindex{eval}

\bifuncindex{execfile}

\bifuncindex{compile}

\section{The \keyword{exec} statement \label{exec}}

\stindex{exec}

\begin{verbatim}

exec_stmt: "exec" expression ["in" expression ["," expression]]

\end{verbatim}

This statement supports dynamic execution of Python code. The first

expression should evaluate to either a string, an open file object, or

a code object. If it is a string, the string is parsed as a suite of

Python statements which is then executed (unless a syntax error

occurs). If it is an open file, the file is parsed until EOF and

executed. If it is a code object, it is simply executed.

In all cases, if the optional parts are omitted, the code is executed

in the current scope. If only the first expression after \keyword{in}

is specified, it should be a dictionary, which will be used for both

the global and the local variables. If two expressions are given,

both must be dictionaries and they are used for the global and local

variables, respectively.

As a side effect, an implementation may insert additional keys into

the dictionaries given besides those corresponding to variable names

set by the executed code. For example, the current implementation

may add a reference to the dictionary of the built-in module

\module{__builtin__} under the key \code{__builtins__} (!).

\ttindex{__builtins__}

\refbimodindex{__builtin__}

\strong{Programmer's hints:}

dynamic evaluation of expressions is supported by the built-in

function \function{eval()}. The built-in functions

\function{globals()} and \function{locals()} return the current global

and local dictionary, respectively, which may be useful to pass around

for use by \keyword{exec}.

\bifuncindex{eval}

\bifuncindex{globals}

\bifuncindex{locals}

Also, in the current implementation, multi-line compound statements must

end with a newline:

\code{exec "for v in seq:\e{}n\e{}tprint v\e{}n"} works, but

\code{exec "for v in seq:\e{}n\e{}tprint v"} fails with

\exception{SyntaxError}.

\exindex{SyntaxError}