.. _simple:

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

Simple statements

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

.. index:: pair: 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:

.. productionlist::

   simple_stmt: `expression_stmt`

              : | `assert_stmt`

              : | `assignment_stmt`

              : | `augmented_assignment_stmt`

              : | `pass_stmt`

              : | `del_stmt`

              : | `print_stmt`

              : | `return_stmt`

              : | `yield_stmt`

              : | `raise_stmt`

              : | `break_stmt`

              : | `continue_stmt`

              : | `import_stmt`

              : | `global_stmt`

              : | `exec_stmt`

.. _exprstmts:

Expression statements

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

.. index::

   pair: expression; statement

   pair: expression; list

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 ``None``).  Other uses of

expression statements are allowed and occasionally useful.  The syntax for an

expression statement is:

.. productionlist::

   expression_stmt: `expression_list`

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

expression).

.. index::

   builtin: repr

   object: None

   pair: string; conversion

   single: output

   pair: standard; output

   pair: writing; values

   pair: procedure; call

In interactive mode, if the value is not ``None``, it is converted to a string

using the built-in :func:`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.)

.. _assignment:

Assignment statements

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

.. index::

   pair: assignment; statement

   pair: binding; name

   pair: rebinding; name

   object: mutable

   pair: attribute; assignment

Assignment statements are used to (re)bind names to values and to modify

attributes or items of mutable objects:

.. productionlist::

   assignment_stmt: (`target_list` "=")+ (`expression_list` | `yield_expression`)

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

   target: `identifier`

         : | "(" `target_list` ")"

         : | "[" `target_list` "]"

         : | `attributeref`

         : | `subscription`

         : | `slicing`

(See section :ref:`primaries` for the syntax definitions for the last three

symbols.)

.. index:: pair: expression; list

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.

.. index::

   single: target

   pair: target; 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:: triple: target; list; assignment

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

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

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

  sequence with the same number of items as 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 ``a, b = "xy"`` is

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

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

* If the target is an identifier (name):

    .. index:: statement: global

  * 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.

  * Otherwise: the name is bound to the object in the current global namespace.

  .. index:: single: destructor

  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 (if it has one) to be called.

* 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.

  .. index:: pair: attribute; assignment

* 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, :exc:`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

  :exc:`AttributeError`).

  .. index::

     pair: subscription; assignment

     object: mutable

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

  evaluated.  It should yield either a mutable sequence object (such as a list) or

  a mapping object (such as a dictionary). Next, the subscript expression is

  evaluated.

  .. index::

     object: sequence

     object: list

  If the primary is a mutable sequence object (such as 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, :exc:`IndexError` is raised

  (assignment to a subscripted sequence cannot add new items to a list).

  .. index::

     object: mapping

     object: dictionary

  If the primary is a mapping object (such as 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).

  .. index:: pair: slicing; assignment

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

  evaluated.  It should yield a mutable sequence object (such as 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.

(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' (for example ``a, b = b, a``

swaps two variables), overlaps *within* the collection of assigned-to variables

are not safe!  For instance, the following program prints ``[0, 2]``::

   x = [0, 1]

   i = 0

   i, x[i] = 1, 2

   print x

.. _augassign:

Augmented assignment statements

-------------------------------

.. index::

   pair: augmented; assignment

   single: statement; assignment, augmented

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

operation and an assignment statement:

.. productionlist::

   augmented_assignment_stmt: `target` `augop` (`expression_list` | `yield_expression`)

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

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

(See section :ref:`primaries` for the syntax definitions for the last three

symbols.)

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

statements, cannot be an unpacking) 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 ``x += 1`` can be rewritten as ``x = x +

1`` to achieve a similar, but not exactly equal effect. In the augmented

version, ``x`` is only evaluated once. Also, when possible, the actual operation

is performed *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

*in-place* behavior, the binary operation performed by augmented assignment is

the same as the normal binary operations.

For targets which are attribute references, the initial value is retrieved with

a :meth:`getattr` and the result is assigned with a :meth:`setattr`.  Notice

that the two methods do not necessarily refer to the same variable.  When

:meth:`getattr` refers to a class variable, :meth:`setattr` still writes to an

instance variable. For example::

   class A:

       x = 3    # class variable

   a = A()

   a.x += 1     # writes a.x as 4 leaving A.x as 3

.. _assert:

The :keyword:`assert` statement

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

.. index::

   statement: assert

   pair: debugging; assertions

Assert statements are a convenient way to insert debugging assertions into a

program:

.. productionlist::

   assert_stmt: "assert" `expression` ["," `expression`]

The simple form, ``assert expression``, is equivalent to ::

   if __debug__:

      if not expression: raise AssertionError

The extended form, ``assert expression1, expression2``, is equivalent to ::

   if __debug__:

      if not expression1: raise AssertionError, expression2

.. index::

   single: __debug__

   exception: AssertionError

These equivalences assume that :const:`__debug__` and :exc:`AssertionError` refer to

the built-in variables with those names.  In the current implementation, the

built-in variable :const:`__debug__` is ``True`` under normal circumstances,

``False`` 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.

Assignments to :const:`__debug__` are illegal.  The value for the built-in variable

is determined when the interpreter starts.

.. _pass:

The :keyword:`pass` statement

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

.. index::

   statement: pass

   pair: null; operation

.. productionlist::

   pass_stmt: "pass"

: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::

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

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

.. _del:

The :keyword:`del` statement

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

.. index::

   statement: del

   pair: deletion; target

   triple: deletion; target; list

.. productionlist::

   del_stmt: "del" `target_list`

Deletion is recursively defined very similar to the way assignment is defined.

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

Deletion of a target list recursively deletes each target, from left to right.

.. index::

   statement: global

   pair: unbinding; name

Deletion of a name removes the binding of that name  from the local or global

namespace, depending on whether the name occurs in a :keyword:`global` statement

in the same code block.  If the name is unbound, a :exc:`NameError` exception

will be raised.

.. index:: pair: free; variable

It is illegal to delete a name from the local namespace if it occurs as a free

variable in a nested block.

.. index:: pair: attribute; deletion

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).

.. _print:

The :keyword:`print` statement

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

.. index:: statement: print

.. productionlist::

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

             : | ">>" `expression` [("," `expression`)+ [","]])

: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 ``'\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.)

.. note::

   Objects which act like file objects but which are not the built-in file objects

   often do not properly emulate this aspect of the file object's behavior, so it

   is best not to rely on this.

.. index::

   single: output

   pair: writing; values

   pair: trailing; comma

   pair: newline; suppression

A ``'\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`.

.. index::

   pair: standard; output

   module: sys

   single: stdout (in module sys)

   exception: RuntimeError

Standard output is defined as the file object named ``stdout`` in the built-in

module :mod:`sys`.  If no such object exists, or if it does not have a

:meth:`write` method, a :exc:`RuntimeError` exception is raised.

.. index:: single: extended print statement

:keyword:`print` also has an extended form, defined by the second portion of the

syntax described above. This form is sometimes referred to as ":keyword:`print`

chevron." In this form, the first expression after the ``>>`` must evaluate to a

"file-like" object, specifically an object that has a :meth:`write` method as

described above.  With this extended form, the subsequent expressions are

printed to this file object.  If the first expression evaluates to ``None``,

then ``sys.stdout`` is used as the file for output.

.. _return:

The :keyword:`return` statement

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

.. index::

   statement: return

   pair: function; definition

   pair: class; definition

.. productionlist::

   return_stmt: "return" [`expression_list`]

:keyword:`return` may only occur syntactically nested in a function definition,

not within a nested class definition.

If an expression list is present, it is evaluated, else ``None`` is substituted.

:keyword:`return` leaves the current function call with the expression list (or

``None``) as return value.

.. index:: keyword: finally

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.

In a generator function, the :keyword:`return` statement is not allowed to

include an :token:`expression_list`.  In that context, a bare :keyword:`return`

indicates that the generator is done and will cause :exc:`StopIteration` to be

raised.

.. _yield:

The :keyword:`yield` statement

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

.. index::

   statement: yield

   single: generator; function

   single: generator; iterator

   single: function; generator

   exception: StopIteration

.. productionlist::

   yield_stmt: `yield_expression`

The :keyword:`yield` statement is only used when defining a generator function,

and is only used in the body of the generator function. Using a :keyword:`yield`

statement in a function definition is sufficient to cause that definition to

create a generator function instead of a normal function.

When a generator function is called, it returns an iterator known as a generator

iterator, or more commonly, a generator.  The body of the generator function is

executed by calling the generator's :meth:`next` method repeatedly until it

raises an exception.

When a :keyword:`yield` statement is executed, the state of the generator is

frozen and the value of :token:`expression_list` is returned to :meth:`next`'s

caller.  By "frozen" we mean that all local state is retained, including the

current bindings of local variables, the instruction pointer, and the internal

evaluation stack: enough information is saved so that the next time :meth:`next`

is invoked, the function can proceed exactly as if the :keyword:`yield`

statement were just another external call.

As of Python version 2.5, the :keyword:`yield` statement is now allowed in the

:keyword:`try` clause of a :keyword:`try` ...  :keyword:`finally` construct.  If

the generator is not resumed before it is finalized (by reaching a zero

reference count or by being garbage collected), the generator-iterator's

:meth:`close` method will be called, allowing any pending :keyword:`finally`

clauses to execute.

.. note::

   In Python 2.2, the :keyword:`yield` statement was only allowed when the

   ``generators`` feature has been enabled.  This ``__future__``

   import statement was used to enable the feature::

      from __future__ import generators

.. seealso::

   :pep:`0255` - Simple Generators

      The proposal for adding generators and the :keyword:`yield` statement to Python.

   :pep:`0342` - Coroutines via Enhanced Generators

      The proposal that, among other generator enhancements, proposed allowing

      :keyword:`yield` to appear inside a :keyword:`try` ... :keyword:`finally` block.

.. _raise:

The :keyword:`raise` statement

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

.. index::

   statement: raise

   single: exception