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[python.git] / Doc / reference / compound_stmts.rst

.. _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` ["," `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.

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

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

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 class name is bound
to this class object in the original local namespace.

**Programmer's note:** Variables defined in the class definition are class
variables; they are shared by all instances.  To create instance variables, they
can be set in a method with ``self.name = value``.  Both class and instance
variables are accessible through the notation "``self.name``", and an instance
variable hides a class variable with the same name when accessed in this way.
Class variables can be used as defaults for instance variables, but using
mutable values there can lead to unexpected results.  For :term:`new-style
class`\es, descriptors can be used to create instance variables with different
implementation details.

Class definitions, like function definitions, may be wrapped by one or
more :term:`decorator` expressions.  The evaluation rules for the
decorator expressions are the same as for functions.  The result must
be a class object, which is then bound to the class name.

.. rubric:: Footnotes

.. [#] The exception is propagated to the invocation stack only if there is no
   :keyword:`finally` clause that negates the exception.

.. [#] 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.