.. _compound:

*******************

Compound statements

*******************

.. index:: pair: 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.

.. index::

   single: clause

   single: suite

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:   ::

   if test1: if test2: print x

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::

   if x < y < z: print x; print y; print z

Summarizing:

.. productionlist::

   compound_stmt: `if_stmt`

                : | `while_stmt`

                : | `for_stmt`

                : | `try_stmt`

                : | `with_stmt`

                : | `funcdef`

                : | `classdef`

                : | `decorated`

   suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT

   statement: `stmt_list` NEWLINE | `compound_stmt`

   stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]

.. index::

   single: NEWLINE token

   single: DEDENT token

   pair: dangling; else

Note that statements always end in a ``NEWLINE`` possibly followed by a

``DEDENT``. 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).

The formatting of the grammar rules in the following sections places each clause

on a separate line for clarity.

.. _if:

.. _elif:

.. _else:

The :keyword:`if` statement

===========================

.. index::

   statement: if

   keyword: elif

   keyword: else

The :keyword:`if` statement is used for conditional execution:

.. productionlist::

   if_stmt: "if" `expression` ":" `suite`

          : ( "elif" `expression` ":" `suite` )*

          : ["else" ":" `suite`]

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.

.. _while:

The :keyword:`while` statement

==============================

.. index::

   statement: while

   pair: loop; statement

   keyword: else

The :keyword:`while` statement is used for repeated execution as long as an

expression is true:

.. productionlist::

   while_stmt: "while" `expression` ":" `suite`

             : ["else" ":" `suite`]

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.

.. index::

   statement: break

   statement: continue

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.

.. _for:

The :keyword:`for` statement

============================

.. index::

   statement: for

   pair: loop; statement

   keyword: in

   keyword: else

   pair: target; list

   object: sequence

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:

.. productionlist::

   for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`

           : ["else" ":" `suite`]

The expression list is evaluated once; it should yield an iterable object.  An

iterator is created for the result of the ``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.

.. index::

   statement: break

   statement: continue

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.

The suite may assign to the variable(s) in the target list; this does not affect

the next item assigned to it.

.. index::

   builtin: range

   pair: Pascal; language

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 :func:`range` returns a sequence of integers suitable to emulate the

effect of Pascal's ``for i := a to b do``; e.g., ``range(3)`` returns the list

``[0, 1, 2]``.

.. warning::

   .. index::

      single: loop; over mutable sequence

      single: mutable sequence; loop over

   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.,

::

   for x in a[:]:

       if x < 0: a.remove(x)

.. _try:

.. _except:

.. _finally:

The :keyword:`try` statement

============================

.. index::

   statement: try

   keyword: except

   keyword: finally

The :keyword:`try` statement specifies exception handlers and/or cleanup code

for a group of statements:

.. productionlist::

   try_stmt: try1_stmt | try2_stmt

   try1_stmt: "try" ":" `suite`

            : ("except" [`expression` [("as" | ",") `target`]] ":" `suite`)+

            : ["else" ":" `suite`]

            : ["finally" ":" `suite`]

   try2_stmt: "try" ":" `suite`

            : "finally" ":" `suite`

.. versionchanged:: 2.5

   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`.

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).

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 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.)

.. index::

   module: sys

   object: traceback

   single: exc_type (in module sys)

   single: exc_value (in module sys)

   single: exc_traceback (in module sys)

Before an except clause's suite is executed, details about the exception are

assigned to three variables in the :mod:`sys` module: ``sys.exc_type`` receives

the object identifying the exception; ``sys.exc_value`` receives the exception's

parameter; ``sys.exc_traceback`` receives a traceback object (see section

:ref:`types`) identifying the point in the program where the exception

occurred. These details are also available through the :func:`sys.exc_info`

function, which returns a tuple ``(exc_type, exc_value, 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.

.. index::

   keyword: else

   statement: return

   statement: break

   statement: continue

The optional :keyword:`else` clause is executed if and when control flows off

the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`

clause are not handled by the preceding :keyword:`except` clauses.

.. index:: keyword: finally

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.

.. index::

   statement: return

   statement: break

   statement: continue

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).

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`.

.. _with:

.. _as:

The :keyword:`with` statement

=============================

.. index:: statement: 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.

.. productionlist::

   with_stmt: "with" `expression` ["as" `target`] ":" `suite`

The execution of the :keyword:`with` statement proceeds as follows:

#. The context expression is evaluated to obtain a context manager.

#. The context manager's :meth:`__enter__` method is invoked.

#. If a target was included in the :keyword:`with` statement, the return value

   from :meth:`__enter__` is assigned to it.

   .. note::

      The :keyword:`with` statement guarantees that if the :meth:`__enter__` method

      returns without an error, then :meth:`__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.

#. The suite is executed.

#. The context manager's :meth:`__exit__` method is invoked. If an exception

   caused the suite to be exited, its type, value, and traceback are passed as

   arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are

   supplied.

   If the suite was exited due to an exception, and the return value from the

   :meth:`__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 :meth:`__exit__` is ignored, and execution proceeds at the normal location

   for the kind of exit that was taken.

.. note::

   In Python 2.5, the :keyword:`with` statement is only allowed when the

   ``with_statement`` feature has been enabled.  It is always enabled in

   Python 2.6.

.. seealso::

   :pep:`0343` - The "with" statement

      The specification, background, and examples for the Python :keyword:`with`

      statement.

.. _function:

.. _def:

Function definitions

====================

.. index::

   statement: def

   pair: function; definition

   pair: function; name

   pair: name; binding

   object: user-defined function

   object: function

A function definition defines a user-defined function object (see section

:ref:`types`):

.. productionlist::

   decorated: decorators (classdef | funcdef)

   decorators: `decorator`+

   decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE

   funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite`

   dotted_name: `identifier` ("." `identifier`)*

   parameter_list: (`defparameter` ",")*

                 : (  "*" `identifier` [, "**" `identifier`]

                 : | "**" `identifier`

                 : | `defparameter` [","] )

   defparameter: `parameter` ["=" `expression`]

   sublist: `parameter` ("," `parameter`)* [","]

   parameter: `identifier` | "(" `sublist` ")"

   funcname: `identifier`

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.

The function definition does not execute the function body; this gets executed

only when the function is called. [#]_

.. index::

  statement: @

A function definition may be wrapped by one or more :term:`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::

   @f1(arg)

   @f2

   def func(): pass

is equivalent to::

   def func(): pass

   func = f1(arg)(f2(func))

.. index:: triple: default; parameter; value

When one or more top-level parameters have the form *parameter* ``=``

*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.

**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 ``None``

as the default, and explicitly test for it in the body of the function, e.g.::

   def whats_on_the_telly(penguin=None):

       if penguin is None:

           penguin = []

       penguin.append("property of the zoo")

       return penguin

.. index::

  statement: *

  statement: **

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 "``*identifier``" is present, it is initialized to a tuple

receiving any excess positional parameters, defaulting to the empty tuple.  If

the form "``**identifier``" is present, it is initialized to a new dictionary

receiving any excess keyword arguments, defaulting to a new empty dictionary.

.. index:: pair: lambda; form

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.

**Programmer's note:** Functions are first-class objects.  A "``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.

.. _class:

Class definitions

=================

.. index::

   object: class

   statement: class

   pair: class; definition

   pair: class; name

   pair: name; binding

   pair: execution; frame

   single: inheritance

   single: docstring

A class definition defines a class object (see section :ref:`types`):

.. productionlist::

   classdef: "class" `classname` [`inheritance`] ":" `suite`

   inheritance: "(" [`expression_list`] ")"

   classname: `identifier`

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 or class type which allows subclassing.  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