\chapter{Simple statements}

\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

| 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}

\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

\verb@None@):

\begin{verbatim}

expression_stmt: condition_list

\end{verbatim}

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

single condition).

\indexii{expression}{list}

In interactive mode, if the value is not \verb@None@, it is converted

to a string using the rules for string conversions (expressions in

reverse quotes), and the resulting string is written to standard

output (see section \ref{print}) on a line by itself.

(The exception for \verb@None@ is made so that procedure calls, which

are syntactically equivalent to expressions, do not cause any output.)

\ttindex{None}

\indexii{string}{conversion}

\index{output}

\indexii{standard}{output}

\indexii{writing}{values}

\indexii{procedure}{call}

\section{Assignment statements}

\indexii{assignment}{statement}

Assignment statements 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 tuple with the same number of items as the list contains

targets, and the items are assigned, from left to right, to the

corresponding targets.

\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 \verb@global@ statement in the current

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

space.

\stindex{global}

\item

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

space.

\end{itemize} % nested

The name is rebound if it was already bound.

\item

If the target is a target list enclosed in parentheses: the object is

assigned to that target list as described above.

\item

If the target is a target list enclosed in square brackets: the object

must be a list with the same number of items as the target list

contains targets, 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, \verb@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 \verb@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

(list) object or a mapping (dictionary) object. Next, the subscript

expression is evaluated.

\indexii{subscription}{assignment}

\obindex{mutable}

If the primary is a mutable sequence object (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, \verb@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 (dictionary) object, 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.

\verb@a, b = b, a@ swaps two variables), overlaps within the

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

following program prints \code@[0, 2]@:

\begin{verbatim}

x = [0, 1]

i = 0

i, x[i] = 1, 2

print x

\end{verbatim}

\section{The {\tt pass} statement}

\stindex{pass}

\begin{verbatim}

pass_stmt: "pass"

\end{verbatim}

\verb@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 {\tt del} statement}

\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 name space, depending on whether the name

occurs in a \verb@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 {\tt print} statement} \label{print}

\stindex{print}

\begin{verbatim}

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

\end{verbatim}

\verb@print@ evaluates each condition 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; or (2) when the last character written to standard

output is \verb/\n/; or (3) when the last write operation on standard

output was not a \verb@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 \verb/"\n"/ character is written at the end, unless the \verb@print@

statement ends with a comma. This is the only action if the statement

contains just the keyword \verb@print@.

\indexii{trailing}{comma}

\indexii{newline}{suppression}

Standard output is defined as the file object named \verb@stdout@

in the built-in module \verb@sys@. If no such object exists,

or if it is not a writable file, a \verb@RuntimeError@ exception is raised.

(The original implementation attempts to write to the system's original

standard output instead, but this is not safe, and should be fixed.)

\indexii{standard}{output}

\bimodindex{sys}

\ttindex{stdout}

\exindex{RuntimeError}

\section{The {\tt return} statement}

\stindex{return}

\begin{verbatim}

return_stmt: "return" [condition_list]

\end{verbatim}

\verb@return@ may only occur syntactically nested in a function

definition, not within a nested class definition.

\indexii{function}{definition}

\indexii{class}{definition}

If a condition list is present, it is evaluated, else \verb@None@

is substituted.

\verb@return@ leaves the current function call with the condition

list (or \verb@None@) as return value.

When \verb@return@ passes control out of a \verb@try@ statement

with a \verb@finally@ clause, that finally clause is executed

before really leaving the function.

\kwindex{finally}

\section{The {\tt raise} statement}

\stindex{raise}

\begin{verbatim}

raise_stmt: "raise" condition ["," condition ["," condition]]

\end{verbatim}

\verb@raise@ evaluates its first condition, which must yield

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

this is evaluated, else \verb@None@ is substituted. If the first

condition is a class object, then the second condition must be an

instance of that class or one of its derivatives. If the first

condition is an instance object, the second condition must be

\verb@None@.

\index{exception}

\indexii{raising}{exception}

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

identified by the first object, with the second one (or \verb@None@)

as its parameter. If the first object is an instance, it raises the

exception identified by the class of the object, with the instance as

its parameter (and there should be no second object, or the second

object should be \verb@None@).

If a third object is present, and it it not \verb@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 {\tt break} statement}

\stindex{break}

\begin{verbatim}

break_stmt: "break"

\end{verbatim}

\verb@break@ may only occur syntactically nested in a \verb@for@

or \verb@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

\verb@else@ clause if the loop has one.

\kwindex{else}

If a \verb@for@ loop is terminated by \verb@break@, the loop control

target keeps its current value.

\indexii{loop control}{target}

When \verb@break@ passes control out of a \verb@try@ statement

with a \verb@finally@ clause, that finally clause is executed

before really leaving the loop.

\kwindex{finally}

\section{The {\tt continue} statement}

\stindex{continue}

\begin{verbatim}

continue_stmt: "continue"

\end{verbatim}

\verb@continue@ may only occur syntactically nested in a \verb@for@ or

\verb@while@ loop, but not nested in a function or class definition or

\verb@try@ statement within that loop.\footnote{Except that it may

currently occur within an {\tt except} clause.}

\stindex{for}

\stindex{while}

\indexii{loop}{statement}

\kwindex{finally}

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

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

\stindex{import}

\begin{verbatim}

import_stmt: "import" identifier ("," identifier)*

| "from" identifier "import" identifier ("," identifier)*

| "from" identifier "import" "*"

\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

name space (of the scope where the \verb@import@ statement occurs).

The first form (without \verb@from@) repeats these steps for each

identifier in the list, the \verb@from@ form performs them once, with

the first identifier specifying the module name.

\indexii{importing}{module}

\indexii{name}{binding}

\kwindex{from}

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

indexed by module name. (The current implementation makes this table

accessible as \[email protected]@.) When a module name is found in

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

definition is started. This first looks for a built-in module

definition, and if no built-in module if the given name is found, it

searches a user-specified list of directories for a file whose name is

the module name with extension \verb@".py"@. (The current

implementation uses the list of strings \[email protected]@ as the search

path; it is initialized from the shell environment variable

\verb@$PYTHONPATH@, with an installation-dependent default.)

\ttindex{modules}

\ttindex{sys.modules}

\indexii{module}{name}

\indexii{built-in}{module}

\indexii{user-defined}{module}

\bimodindex{sys}

\ttindex{path}

\ttindex{sys.path}

\indexii{filename}{extension}

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,

\verb@ImportError@ is raised. If a file is found, it is parsed,

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

\verb@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 \verb@import@ statement binds the module name in the

local name space to the module object, and then goes on to import the

next identifier, if any. The \verb@from@ from 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 name space to the object thus found. If a name is not found,

\verb@ImportError@ is raised. If the list of identifiers is replaced

by a star (\verb@*@), all names defined in the module are bound,

except those beginning with an underscore(\verb@_@).

\indexii{name}{binding}

\exindex{ImportError}

Names bound by import statements may not occur in \verb@global@

statements in the same scope.

\stindex{global}

The \verb@from@ form with \verb@*@ may only occur in a module scope.

\kwindex{from}

\ttindex{from ... import *}

(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.)

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

\stindex{global}

\begin{verbatim}

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

\end{verbatim}

The \verb@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 {\em using} global names is automatic

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

names would be impossible without \verb@global@.

\indexiii{global}{name}{binding}

Names listed in a \verb@global@ statement must not be used in the same

code block before that \verb@global@ statement is executed.

Names listed in a \verb@global@ statement must not be defined as formal

parameters or in a \verb@for@ loop control target, \verb@class@

definition, function definition, or \verb@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.)

Note: the \verb@global@ is a directive to the parser. Therefore, it

applies only to code parsed at the same time as the \verb@global@

statement. In particular, a \verb@global@ statement contained in an

\verb@exec@ statement does not affect the code block {\em containing}

the \verb@exec@ statement, and code contained in an \verb@exec@

statement is unaffected by \verb@global@ statements in the code

containing the \verb@exec@ statement. The same applies to the

\verb@eval()@, \verb@execfie()@ and \verb@compile()@ functions.

\stindex{exec}

\ttindex{eval}

\ttindex{execfile}

\ttindex{compile}