path: root/Doc/ref/ref6.tex

\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
              | 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}


\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}

\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 ("," module)* 
              | "from" module "import" identifier ("," identifier)*
              | "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.  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.  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}