path: root/Doc/reference/expressions.rst

.. _expressions:

***********
Expressions
***********

.. index:: expression, BNF

This chapter explains the meaning of the elements of expressions in Python.

**Syntax Notes:** In this and the following chapters, extended BNF notation will
be used to describe syntax, not lexical analysis.  When (one alternative of) a
syntax rule has the form

.. productionlist:: *
   name: `othername`

and no semantics are given, the semantics of this form of ``name`` are the same
as for ``othername``.


.. _conversions:

Arithmetic conversions
======================

.. index:: pair: arithmetic; conversion

When a description of an arithmetic operator below uses the phrase "the numeric
arguments are converted to a common type," this means that the operator
implementation for built-in types works that way:

* If either argument is a complex number, the other is converted to complex;

* otherwise, if either argument is a floating point number, the other is
  converted to floating point;

* otherwise, both must be integers and no conversion is necessary.

Some additional rules apply for certain operators (e.g., a string left argument
to the '%' operator).  Extensions must define their own conversion behavior.


.. _atoms:

Atoms
=====

.. index:: atom

Atoms are the most basic elements of expressions.  The simplest atoms are
identifiers or literals.  Forms enclosed in parentheses, brackets or braces are
also categorized syntactically as atoms.  The syntax for atoms is:

.. productionlist::
   atom: `identifier` | `literal` | `enclosure`
   enclosure: `parenth_form` | `list_display` | `dict_display` | `set_display`
            : | `generator_expression` | `yield_atom`


.. _atom-identifiers:

Identifiers (Names)
-------------------

.. index:: name, identifier

An identifier occurring as an atom is a name.  See section :ref:`identifiers`
for lexical definition and section :ref:`naming` for documentation of naming and
binding.

.. index:: exception: NameError

When the name is bound to an object, evaluation of the atom yields that object.
When a name is not bound, an attempt to evaluate it raises a :exc:`NameError`
exception.

.. index::
   pair: name; mangling
   pair: private; names

**Private name mangling:** When an identifier that textually occurs in a class
definition begins with two or more underscore characters and does not end in two
or more underscores, it is considered a :dfn:`private name` of that class.
Private names are transformed to a longer form before code is generated for
them.  The transformation inserts the class name in front of the name, with
leading underscores removed, and a single underscore inserted in front of the
class name.  For example, the identifier ``__spam`` occurring in a class named
``Ham`` will be transformed to ``_Ham__spam``.  This transformation is
independent of the syntactical context in which the identifier is used.  If the
transformed name is extremely long (longer than 255 characters), implementation
defined truncation may happen.  If the class name consists only of underscores,
no transformation is done.


.. _atom-literals:

Literals
--------

.. index:: single: literal

Python supports string and bytes literals and various numeric literals:

.. productionlist::
   literal: `stringliteral` | `bytesliteral`
          : | `integer` | `floatnumber` | `imagnumber`

Evaluation of a literal yields an object of the given type (string, bytes,
integer, floating point number, complex number) with the given value.  The value
may be approximated in the case of floating point and imaginary (complex)
literals.  See section :ref:`literals` for details.

.. index::
   triple: immutable; data; type
   pair: immutable; object

With the exception of bytes literals, these all correspond to immutable data
types, and hence the object's identity is less important than its value.
Multiple evaluations of literals with the same value (either the same occurrence
in the program text or a different occurrence) may obtain the same object or a
different object with the same value.


.. _parenthesized:

Parenthesized forms
-------------------

.. index:: single: parenthesized form

A parenthesized form is an optional expression list enclosed in parentheses:

.. productionlist::
   parenth_form: "(" [`expression_list`] ")"

A parenthesized expression list yields whatever that expression list yields: if
the list contains at least one comma, it yields a tuple; otherwise, it yields
the single expression that makes up the expression list.

.. index:: pair: empty; tuple

An empty pair of parentheses yields an empty tuple object.  Since tuples are
immutable, the rules for literals apply (i.e., two occurrences of the empty
tuple may or may not yield the same object).

.. index::
   single: comma
   pair: tuple; display

Note that tuples are not formed by the parentheses, but rather by use of the
comma operator.  The exception is the empty tuple, for which parentheses *are*
required --- allowing unparenthesized "nothing" in expressions would cause
ambiguities and allow common typos to pass uncaught.


.. _comprehensions:

Displays for lists, sets and dictionaries
-----------------------------------------

For constructing a list, a set or a dictionary Python provides special syntax
called "displays", each of them in two flavors:

* either the container contents are listed explicitly, or

* they are computed via a set of looping and filtering instructions, called a
  :dfn:`comprehension`.

Common syntax elements for comprehensions are:

.. productionlist::
   comprehension: `expression` `comp_for`
   comp_for: "for" `target_list` "in" `or_test` [`comp_iter`]
   comp_iter: `comp_for` | `comp_if`
   comp_if: "if" `expression_nocond` [`comp_iter`]

The comprehension consists of a single expression followed by at least one
:keyword:`for` clause and zero or more :keyword:`for` or :keyword:`if` clauses.
In this case, the elements of the new container are those that would be produced
by considering each of the :keyword:`for` or :keyword:`if` clauses a block,
nesting from left to right, and evaluating the expression to produce an element
each time the innermost block is reached.

Note that the comprehension is executed in a separate scope, so names assigned
to in the target list don't "leak" in the enclosing scope.


.. _lists:

List displays
-------------

.. index::
   pair: list; display
   pair: list; comprehensions
   pair: empty; list
   object: list

A list display is a possibly empty series of expressions enclosed in square
brackets:

.. productionlist::
   list_display: "[" [`expression_list` | `comprehension`] "]"

A list display yields a new list object, the contents being specified by either
a list of expressions or a comprehension.  When a comma-separated list of
expressions is supplied, its elements are evaluated from left to right and
placed into the list object in that order.  When a comprehension is supplied,
the list is constructed from the elements resulting from the comprehension.


.. _set:

Set displays
------------

.. index:: pair: set; display
           object: set

A set display is denoted by curly braces and distinguishable from dictionary
displays by the lack of colons separating keys and values:

.. productionlist::
   set_display: "{" [`expression_list` | `comprehension`] "}"

A set display yields a new mutable set object, the contents being specified by
either a sequence of expressions or a comprehension.  When a comma-separated
list of expressions is supplied, its elements are evaluated from left to right
and added to the set object.  When a comprehension is supplied, the set is
constructed from the elements resulting from the comprehension.


Variables used in the generator expression are evaluated lazily in a separate
scope when the :meth:`next` method is called for the generator object (in the
same fashion as for normal generators).  However, the :keyword:`in` expression
of the leftmost :keyword:`for` clause is immediately evaluated in the current
scope so that an error produced by it can be seen before any other possible
error in the code that handles the generator expression.  Subsequent
:keyword:`for` and :keyword:`if` clauses cannot be evaluated immediately since
they may depend on the previous :keyword:`for` loop.  For example:
``(x*y for x in range(10) for y in bar(x))``.

The parentheses can be omitted on calls with only one argument. See section
:ref:`calls` for the detail.


.. _dict:

Dictionary displays
-------------------

.. index:: pair: dictionary; display
           key, datum, key/datum pair
           object: dictionary

A dictionary display is a possibly empty series of key/datum pairs enclosed in
curly braces:

.. productionlist::
   dict_display: "{" [`key_datum_list` | `dict_comprehension`] "}"
   key_datum_list: `key_datum` ("," `key_datum`)* [","]
   key_datum: `expression` ":" `expression`
   dict_comprehension: `expression` ":" `expression` `comp_for`

A dictionary display yields a new dictionary object.

If a comma-separated sequence of key/datum pairs is given, they are evaluated
from left to right to define the entries of the dictionary: each key object is
used as a key into the dictionary to store the corresponding datum.  This means
that you can specify the same key multiple times in the key/datum list, and the
final dictionary's value for that key will be the last one given.

A dict comprehension, in contrast to list and set comprehensions, needs two
expressions separated with a colon followed by the usual "for" and "if" clauses.
When the comprehension is run, the resulting key and value elements are inserted
in the new dictionary in the order they are produced.

.. index:: pair: immutable; object
           hashable

Restrictions on the types of the key values are listed earlier in section
:ref:`types`.  (To summarize, the key type should be :term:`hashable`, which excludes
all mutable objects.)  Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key value
prevails.


.. _genexpr:

Generator expressions
---------------------

.. index:: pair: generator; expression
           object: generator

A generator expression is a compact generator notation in parentheses:

.. productionlist::
   generator_expression: "(" `expression` `comp_for` ")"

A generator expression yields a new generator object.  Its syntax is the same as
for comprehensions, except that it is enclosed in parentheses instead of
brackets or curly braces.

Variables used in the generator expression are evaluated lazily when the
:meth:`__next__` method is called for generator object (in the same fashion as
normal generators).  However, the leftmost :keyword:`for` clause is immediately
evaluated, so that an error produced by it can be seen before any other possible
error in the code that handles the generator expression.  Subsequent
:keyword:`for` clauses cannot be evaluated immediately since they may depend on
the previous :keyword:`for` loop. For example: ``(x*y for x in range(10) for y
in bar(x))``.

The parentheses can be omitted on calls with only one argument.  See section
:ref:`calls` for the detail.


.. _yieldexpr:

Yield expressions
-----------------

.. index::
   keyword: yield
   pair: yield; expression
   pair: generator; function

.. productionlist::
   yield_atom: "(" `yield_expression` ")"
   yield_expression: "yield" [`expression_list`]

The :keyword:`yield` expression is only used when defining a generator function,
and can only be used in the body of a function definition.  Using a
:keyword:`yield` expression 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.  That generator then controls the execution of a generator function.
The execution starts when one of the generator's methods is called.  At that
time, the execution proceeds to the first :keyword:`yield` expression, where it
is suspended again, returning the value of :token:`expression_list` to
generator's caller.  By suspended we mean that all local state is retained,
including the current bindings of local variables, the instruction pointer, and
the internal evaluation stack.  When the execution is resumed by calling one of
the generator's methods, the function can proceed exactly as if the
:keyword:`yield` expression was just another external call.  The value of the
:keyword:`yield` expression after resuming depends on the method which resumed
the execution.

.. index:: single: coroutine

All of this makes generator functions quite similar to coroutines; they yield
multiple times, they have more than one entry point and their execution can be
suspended.  The only difference is that a generator function cannot control
where should the execution continue after it yields; the control is always
transfered to the generator's caller.

The :keyword:`yield` statement is 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.

.. index:: object: generator

The following generator's methods can be used to control the execution of a
generator function:

.. index:: exception: StopIteration


.. method:: generator.__next__()

   Starts the execution of a generator function or resumes it at the last
   executed :keyword:`yield` expression.  When a generator function is resumed
   with a :meth:`next` method, the current :keyword:`yield` expression always
   evaluates to :const:`None`.  The execution then continues to the next
   :keyword:`yield` expression, where the generator is suspended again, and the
   value of the :token:`expression_list` is returned to :meth:`next`'s caller.
   If the generator exits without yielding another value, a :exc:`StopIteration`
   exception is raised.

   This method is normally called implicitly, e.g. by a :keyword:`for` loop, or
   by the built-in :func:`next` function.


.. method:: generator.send(value)

   Resumes the execution and "sends" a value into the generator function.  The
   ``value`` argument becomes the result of the current :keyword:`yield`
   expression.  The :meth:`send` method returns the next value yielded by the
   generator, or raises :exc:`StopIteration` if the generator exits without
   yielding another value.  When :meth:`send` is called to start the generator,
   it must be called with :const:`None` as the argument, because there is no
   :keyword:`yield` expression that could receive the value.


.. method:: generator.throw(type[, value[, traceback]])

   Raises an exception of type ``type`` at the point where generator was paused,
   and returns the next value yielded by the generator function.  If the generator
   exits without yielding another value, a :exc:`StopIteration` exception is
   raised.  If the generator function does not catch the passed-in exception, or
   raises a different exception, then that exception propagates to the caller.

.. index:: exception: GeneratorExit


.. method:: generator.close()

   Raises a :exc:`GeneratorExit` at the point where the generator function was
   paused.  If the generator function then raises :exc:`StopIteration` (by
   exiting normally, or due to already being closed) or :exc:`GeneratorExit` (by
   not catching the exception), close returns to its caller.  If the generator
   yields a value, a :exc:`RuntimeError` is raised.  If the generator raises any
   other exception, it is propagated to the caller.  :meth:`close` does nothing
   if the generator has already exited due to an exception or normal exit.

Here is a simple example that demonstrates the behavior of generators and
generator functions::

   >>> def echo(value=None):
   ...     print("Execution starts when 'next()' is called for the first time.")
   ...     try:
   ...         while True:
   ...             try:
   ...                 value = (yield value)
   ...             except Exception, e:
   ...                 value = e
   ...     finally:
   ...         print("Don't forget to clean up when 'close()' is called.")
   ...
   >>> generator = echo(1)
   >>> print(next(generator))
   Execution starts when 'next()' is called for the first time.
   1
   >>> print(next(generator))
   None
   >>> print(generator.send(2))
   2
   >>> generator.throw(TypeError, "spam")
   TypeError('spam',)
   >>> generator.close()
   Don't forget to clean up when 'close()' is called.


.. 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 to enhance the API and syntax of generators, making them
      usable as simple coroutines.


.. _primaries:

Primaries
=========

.. index:: single: primary

Primaries represent the most tightly bound operations of the language. Their
syntax is:

.. productionlist::
   primary: `atom` | `attributeref` | `subscription` | `slicing` | `call`


.. _attribute-references:

Attribute references
--------------------

.. index:: pair: attribute; reference

An attribute reference is a primary followed by a period and a name:

.. productionlist::
   attributeref: `primary` "." `identifier`

.. index::
   exception: AttributeError
   object: module
   object: list

The primary must evaluate to an object of a type that supports attribute
references, which most objects do.  This object is then asked to produce the
attribute whose name is the identifier (which can be customized by overriding
the :meth:`__getattr__` method).  If this attribute is not available, the
exception :exc:`AttributeError` is raised.  Otherwise, the type and value of the
object produced is determined by the object.  Multiple evaluations of the same
attribute reference may yield different objects.


.. _subscriptions:

Subscriptions
-------------

.. index:: single: subscription

.. index::
   object: sequence
   object: mapping
   object: string
   object: tuple
   object: list
   object: dictionary
   pair: sequence; item

A subscription selects an item of a sequence (string, tuple or list) or mapping
(dictionary) object:

.. productionlist::
   subscription: `primary` "[" `expression_list` "]"

The primary must evaluate to an object that supports subscription, e.g. a list
or dictionary.  User-defined objects can support subscription by defining a
:meth:`__getitem__` method.

For built-in objects, there are two types of objects that support subscription:

If the primary is a mapping, the expression list must evaluate to an object
whose value is one of the keys of the mapping, and the subscription selects the
value in the mapping that corresponds to that key.  (The expression list is a
tuple except if it has exactly one item.)

If the primary is a sequence, the expression (list) must evaluate to an integer.
If this value is negative, the length of the sequence is added to it (so that,
e.g., ``x[-1]`` selects the last item of ``x``.)  The resulting value must be a
nonnegative integer less than the number of items in the sequence, and the
subscription selects the item whose index is that value (counting from zero).

.. index::
   single: character
   pair: string; item

A string's items are characters.  A character is not a separate data type but a
string of exactly one character.


.. _slicings:

Slicings
--------

.. index::
   single: slicing
   single: slice

.. index::
   object: sequence
   object: string
   object: tuple
   object: list

A slicing selects a range of items in a sequence object (e.g., a string, tuple
or list).  Slicings may be used as expressions or as targets in assignment or
:keyword:`del` statements.  The syntax for a slicing:

.. productionlist::
   slicing: `primary` "[" `slice_list` "]" 
   slice_list: `slice_item` ("," `slice_item`)* [","]
   slice_item: `expression` | `proper_slice`
   proper_slice: [`lower_bound`] ":" [`upper_bound`] [ ":" [`stride`] ]
   lower_bound: `expression`
   upper_bound: `expression`
   stride: `expression`

There is ambiguity in the formal syntax here: anything that looks like an
expression list also looks like a slice list, so any subscription can be
interpreted as a slicing.  Rather than further complicating the syntax, this is
disambiguated by defining that in this case the interpretation as a subscription
takes priority over the interpretation as a slicing (this is the case if the
slice list contains no proper slice).

.. index::
   single: start (slice object attribute)
   single: stop (slice object attribute)
   single: step (slice object attribute)

The semantics for a slicing are as follows.  The primary must evaluate to a
mapping object, and it is indexed (using the same :meth:`__getitem__` method as
normal subscription) with a key that is constructed from the slice list, as
follows.  If the slice list contains at least one comma, the key is a tuple
containing the conversion of the slice items; otherwise, the conversion of the
lone slice item is the key.  The conversion of a slice item that is an
expression is that expression.  The conversion of a proper slice is a slice
object (see section :ref:`types`) whose :attr:`start`, :attr:`stop` and
:attr:`step` attributes are the values of the expressions given as lower bound,
upper bound and stride, respectively, substituting ``None`` for missing
expressions.


.. _calls:

Calls
-----

.. index:: single: call

.. index:: object: callable

A call calls a callable object (e.g., a function) with a possibly empty series
of arguments:

.. productionlist::
   call: `primary` "(" [`argument_list` [","]
       : | `expression` `genexpr_for`] ")"
   argument_list: `positional_arguments` ["," `keyword_arguments`]
                : ["," "*" `expression`]
                : ["," "**" `expression`]
                : | `keyword_arguments` ["," "*" `expression`]
                : ["," "**" `expression`]
                : | "*" `expression` ["," "**" `expression`]
                : | "**" `expression`
   positional_arguments: `expression` ("," `expression`)*
   keyword_arguments: `keyword_item` ("," `keyword_item`)*
   keyword_item: `identifier` "=" `expression`

A trailing comma may be present after the positional and keyword arguments but
does not affect the semantics.

The primary must evaluate to a callable object (user-defined functions, built-in
functions, methods of built-in objects, class objects, methods of class
instances, and all objects having a :meth:`__call__` method are callable).  All
argument expressions are evaluated before the call is attempted.  Please refer
to section :ref:`function` for the syntax of formal parameter lists.

.. XXX update with kwonly args PEP

If keyword arguments are present, they are first converted to positional
arguments, as follows.  First, a list of unfilled slots is created for the
formal parameters.  If there are N positional arguments, they are placed in the
first N slots.  Next, for each keyword argument, the identifier is used to
determine the corresponding slot (if the identifier is the same as the first
formal parameter name, the first slot is used, and so on).  If the slot is
already filled, a :exc:`TypeError` exception is raised. Otherwise, the value of
the argument is placed in the slot, filling it (even if the expression is
``None``, it fills the slot).  When all arguments have been processed, the slots
that are still unfilled are filled with the corresponding default value from the
function definition.  (Default values are calculated, once, when the function is
defined; thus, a mutable object such as a list or dictionary used as default
value will be shared by all calls that don't specify an argument value for the
corresponding slot; this should usually be avoided.)  If there are any unfilled
slots for which no default value is specified, a :exc:`TypeError` exception is
raised.  Otherwise, the list of filled slots is used as the argument list for
the call.

If there are more positional arguments than there are formal parameter slots, a
:exc:`TypeError` exception is raised, unless a formal parameter using the syntax
``*identifier`` is present; in this case, that formal parameter receives a tuple
containing the excess positional arguments (or an empty tuple if there were no
excess positional arguments).

If any keyword argument does not correspond to a formal parameter name, a
:exc:`TypeError` exception is raised, unless a formal parameter using the syntax
``**identifier`` is present; in this case, that formal parameter receives a
dictionary containing the excess keyword arguments (using the keywords as keys
and the argument values as corresponding values), or a (new) empty dictionary if
there were no excess keyword arguments.

If the syntax ``*expression`` appears in the function call, ``expression`` must
evaluate to a sequence.  Elements from this sequence are treated as if they were
additional positional arguments; if there are positional arguments *x1*,...,*xN*
, and ``expression`` evaluates to a sequence *y1*,...,*yM*, this is equivalent
to a call with M+N positional arguments *x1*,...,*xN*,*y1*,...,*yM*.

A consequence of this is that although the ``*expression`` syntax appears
*after* any keyword arguments, it is processed *before* the keyword arguments
(and the ``**expression`` argument, if any -- see below).  So::

   >>> def f(a, b):
   ...  print(a, b)
   ...
   >>> f(b=1, *(2,))
   2 1
   >>> f(a=1, *(2,))
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: f() got multiple values for keyword argument 'a'
   >>> f(1, *(2,))
   1 2

It is unusual for both keyword arguments and the ``*expression`` syntax to be
used in the same call, so in practice this confusion does not arise.

If the syntax ``**expression`` appears in the function call, ``expression`` must
evaluate to a mapping, the contents of which are treated as additional keyword
arguments.  In the case of a keyword appearing in both ``expression`` and as an
explicit keyword argument, a :exc:`TypeError` exception is raised.

Formal parameters using the syntax ``*identifier`` or ``**identifier`` cannot be
used as positional argument slots or as keyword argument names.

A call always returns some value, possibly ``None``, unless it raises an
exception.  How this value is computed depends on the type of the callable
object.

If it is---

a user-defined function:
   .. index::
      pair: function; call
      triple: user-defined; function; call
      object: user-defined function
      object: function

   The code block for the function is executed, passing it the argument list.  The
   first thing the code block will do is bind the formal parameters to the
   arguments; this is described in section :ref:`function`.  When the code block
   executes a :keyword:`return` statement, this specifies the return value of the
   function call.

a built-in function or method:
   .. index::
      pair: function; call
      pair: built-in function; call
      pair: method; call
      pair: built-in method; call
      object: built-in method
      object: built-in function
      object: method
      object: function

   The result is up to the interpreter; see :ref:`built-in-funcs` for the
   descriptions of built-in functions and methods.

a class object:
   .. index::
      object: class
      pair: class object; call

   A new instance of that class is returned.

a class instance method:
   .. index::
      object: class instance
      object: instance
      pair: class instance; call

   The corresponding user-defined function is called, with an argument list that is
   one longer than the argument list of the call: the instance becomes the first
   argument.

a class instance:
   .. index::
      pair: instance; call
      single: __call__() (object method)

   The class must define a :meth:`__call__` method; the effect is then the same as
   if that method was called.


.. _power:

The power operator
==================

The power operator binds more tightly than unary operators on its left; it binds
less tightly than unary operators on its right.  The syntax is:

.. productionlist::
   power: `primary` ["**" `u_expr`]

Thus, in an unparenthesized sequence of power and unary operators, the operators
are evaluated from right to left (this does not constrain the evaluation order
for the operands): ``-1**2`` results in ``-1``.

The power operator has the same semantics as the built-in :func:`pow` function,
when called with two arguments: it yields its left argument raised to the power
of its right argument.  The numeric arguments are first converted to a common
type, and the result is of that type.

For int operands, the result has the same type as the operands unless the second
argument is negative; in that case, all arguments are converted to float and a
float result is delivered. For example, ``10**2`` returns ``100``, but
``10**-2`` returns ``0.01``.

Raising ``0.0`` to a negative power results in a :exc:`ZeroDivisionError`.
Raising a negative number to a fractional power results in a :class:`complex`
number. (In earlier versions it raised a :exc:`ValueError`.)


.. _unary:

Unary arithmetic operations
===========================

.. index::
   triple: unary; arithmetic; operation
   triple: unary; bitwise; operation

All unary arithmetic (and bitwise) operations have the same priority:

.. productionlist::
   u_expr: `power` | "-" `u_expr` | "+" `u_expr` | "~" `u_expr`

.. index::
   single: negation
   single: minus

The unary ``-`` (minus) operator yields the negation of its numeric argument.

.. index:: single: plus

The unary ``+`` (plus) operator yields its numeric argument unchanged.

.. index:: single: inversion


The unary ``~`` (invert) operator yields the bitwise inversion of its plain or
long integer argument.  The bitwise inversion of ``x`` is defined as
``-(x+1)``.  It only applies to integral numbers.

.. index:: exception: TypeError

In all three cases, if the argument does not have the proper type, a
:exc:`TypeError` exception is raised.


.. _binary:

Binary arithmetic operations
============================

.. index:: triple: binary; arithmetic; operation

The binary arithmetic operations have the conventional priority levels.  Note
that some of these operations also apply to certain non-numeric types.  Apart
from the power operator, there are only two levels, one for multiplicative
operators and one for additive operators:

.. productionlist::
   m_expr: `u_expr` | `m_expr` "*" `u_expr` | `m_expr` "//" `u_expr` | `m_expr` "/" `u_expr`
         : | `m_expr` "%" `u_expr`
   a_expr: `m_expr` | `a_expr` "+" `m_expr` | `a_expr` "-" `m_expr`

.. index:: single: multiplication

The ``*`` (multiplication) operator yields the product of its arguments.  The
arguments must either both be numbers, or one argument must be an integer and
the other must be a sequence. In the former case, the numbers are converted to a
common type and then multiplied together.  In the latter case, sequence
repetition is performed; a negative repetition factor yields an empty sequence.

.. index::
   exception: ZeroDivisionError
   single: division

The ``/`` (division) and ``//`` (floor division) operators yield the quotient of
their arguments.  The numeric arguments are first converted to a common type.
Integer division yields a float, while floor division of integers results in an
integer; the result is that of mathematical division with the 'floor' function
applied to the result.  Division by zero raises the :exc:`ZeroDivisionError`
exception.

.. index:: single: modulo

The ``%`` (modulo) operator yields the remainder from the division of the first
argument by the second.  The numeric arguments are first converted to a common
type.  A zero right argument raises the :exc:`ZeroDivisionError` exception.  The
arguments may be floating point numbers, e.g., ``3.14%0.7`` equals ``0.34``
(since ``3.14`` equals ``4*0.7 + 0.34``.)  The modulo operator always yields a
result with the same sign as its second operand (or zero); the absolute value of
the result is strictly smaller than the absolute value of the second operand
[#]_.

The floor division and modulo operators are connected by the following
identity: ``x == (x//y)*y + (x%y)``.  Floor division and modulo are also
connected with the built-in function :func:`divmod`: ``divmod(x, y) == (x//y,
x%y)``. [#]_.

In addition to performing the modulo operation on numbers, the ``%`` operator is
also overloaded by string objects to perform old-style string formatting (also
known as interpolation).  The syntax for string formatting is described in the
Python Library Reference, section :ref:`old-string-formatting`.

The floor division operator, the modulo operator, and the :func:`divmod`
function are not defined for complex numbers.  Instead, convert to a floating
point number using the :func:`abs` function if appropriate.

.. index:: single: addition

The ``+`` (addition) operator yields the sum of its arguments.  The arguments
must either both be numbers or both sequences of the same type.  In the former
case, the numbers are converted to a common type and then added together.  In
the latter case, the sequences are concatenated.

.. index:: single: subtraction

The ``-`` (subtraction) operator yields the difference of its arguments.  The
numeric arguments are first converted to a common type.


.. _shifting:

Shifting operations
===================

.. index:: pair: shifting; operation

The shifting operations have lower priority than the arithmetic operations:

.. productionlist::
   shift_expr: `a_expr` | `shift_expr` ( "<<" | ">>" ) `a_expr`

These operators accept integers as arguments.  They shift the first argument to
the left or right by the number of bits given by the second argument.

.. index:: exception: ValueError

A right shift by *n* bits is defined as division by ``pow(2,n)``.  A left shift
by *n* bits is defined as multiplication with ``pow(2,n)``.


.. _bitwise:

Binary bitwise operations
=========================

.. index:: triple: binary; bitwise; operation

Each of the three bitwise operations has a different priority level:

.. productionlist::
   and_expr: `shift_expr` | `and_expr` "&" `shift_expr`
   xor_expr: `and_expr` | `xor_expr` "^" `and_expr`
   or_expr: `xor_expr` | `or_expr` "|" `xor_expr`

.. index:: pair: bitwise; and

The ``&`` operator yields the bitwise AND of its arguments, which must be
integers.

.. index::
   pair: bitwise; xor
   pair: exclusive; or

The ``^`` operator yields the bitwise XOR (exclusive OR) of its arguments, which
must be integers.

.. index::
   pair: bitwise; or
   pair: inclusive; or

The ``|`` operator yields the bitwise (inclusive) OR of its arguments, which
must be integers.


.. _comparisons:
.. _is:
.. _isnot:
.. _in:
.. _notin:

Comparisons
===========

.. index:: single: comparison

.. index:: pair: C; language

Unlike C, all comparison operations in Python have the same priority, which is
lower than that of any arithmetic, shifting or bitwise operation.  Also unlike
C, expressions like ``a < b < c`` have the interpretation that is conventional
in mathematics:

.. productionlist::
   comparison: `or_expr` ( `comp_operator` `or_expr` )*
   comp_operator: "<" | ">" | "==" | ">=" | "<=" | "!="
                : | "is" ["not"] | ["not"] "in"

Comparisons yield boolean values: ``True`` or ``False``.

.. index:: pair: chaining; comparisons

Comparisons can be chained arbitrarily, e.g., ``x < y <= z`` is equivalent to
``x < y and y <= z``, except that ``y`` is evaluated only once (but in both
cases ``z`` is not evaluated at all when ``x < y`` is found to be false).

Formally, if *a*, *b*, *c*, ..., *y*, *z* are expressions and *op1*, *op2*, ...,
*opN* are comparison operators, then ``a op1 b op2 c ... y opN z`` is equivalent
to ``a op1 b and b op2 c and ... y opN z``, except that each expression is
evaluated at most once.

Note that ``a op1 b op2 c`` doesn't imply any kind of comparison between *a* and
*c*, so that, e.g., ``x < y > z`` is perfectly legal (though perhaps not
pretty).

The operators ``<``, ``>``, ``==``, ``>=``, ``<=``, and ``!=`` compare the
values of two objects.  The objects need not have the same type. If both are
numbers, they are converted to a common type.  Otherwise, objects of different
types *always* compare unequal, and are ordered consistently but arbitrarily.
You can control comparison behavior of objects of non-builtin types by defining
a :meth:`__cmp__` method or rich comparison methods like :meth:`__gt__`,
described in section :ref:`specialnames`.

(This unusual definition of comparison was used to simplify the definition of
operations like sorting and the :keyword:`in` and :keyword:`not in` operators.
In the future, the comparison rules for objects of different types are likely to
change.)

Comparison of objects of the same type depends on the type:

* Numbers are compared arithmetically.

* Bytes objects are compared lexicographically using the numeric values of their
  elements.

* Strings are compared lexicographically using the numeric equivalents (the
  result of the built-in function :func:`ord`) of their characters. [#]_ String
  and bytes object can't be compared!

* Tuples and lists are compared lexicographically using comparison of
  corresponding elements.  This means that to compare equal, each element must
  compare equal and the two sequences must be of the same type and have the same
  length.

  If not equal, the sequences are ordered the same as their first differing
  elements.  For example, ``cmp([1,2,x], [1,2,y])`` returns the same as
  ``cmp(x,y)``.  If the corresponding element does not exist, the shorter
  sequence is ordered first (for example, ``[1,2] < [1,2,3]``).

* Mappings (dictionaries) compare equal if and only if their sorted ``(key,
  value)`` lists compare equal. [#]_ Outcomes other than equality are resolved
  consistently, but are not otherwise defined. [#]_

* Most other objects of builtin types compare unequal unless they are the same
  object; the choice whether one object is considered smaller or larger than
  another one is made arbitrarily but consistently within one execution of a
  program.

The operators :keyword:`in` and :keyword:`not in` test for membership.  ``x in
s`` evaluates to true if *x* is a member of *s*, and false otherwise.  ``x not
in s`` returns the negation of ``x in s``.  All built-in sequences and set types
support this as well as dictionary, for which :keyword:`in` tests whether a the
dictionary has a given key.

For the list and tuple types, ``x in y`` is true if and only if there exists an
index *i* such that ``x == y[i]`` is true.

For the string and bytes types, ``x in y`` is true if and only if *x* is a
substring of *y*.  An equivalent test is ``y.find(x) != -1``.  Empty strings are
always considered to be a substring of any other string, so ``"" in "abc"`` will
return ``True``.

For user-defined classes which define the :meth:`__contains__` method, ``x in
y`` is true if and only if ``y.__contains__(x)`` is true.

For user-defined classes which do not define :meth:`__contains__` and do define
:meth:`__getitem__`, ``x in y`` is true if and only if there is a non-negative
integer index *i* such that ``x == y[i]``, and all lower integer indices do not
raise :exc:`IndexError` exception.  (If any other exception is raised, it is as
if :keyword:`in` raised that exception).

.. index::
   operator: in
   operator: not in
   pair: membership; test
   object: sequence

The operator :keyword:`not in` is defined to have the inverse true value of
:keyword:`in`.

.. index::
   operator: is
   operator: is not
   pair: identity; test

The operators :keyword:`is` and :keyword:`is not` test for object identity: ``x
is y`` is true if and only if *x* and *y* are the same object.  ``x is not y``
yields the inverse truth value.


.. _booleans:
.. _and:
.. _or:
.. _not:

Boolean operations
==================

.. index::
   pair: Conditional; expression
   pair: Boolean; operation

Boolean operations have the lowest priority of all Python operations:

.. productionlist::
   expression: `conditional_expression` | `lambda_form`
   expression_nocond: `or_test` | `lambda_form_nocond`
   conditional_expression: `or_test` ["if" `or_test` "else" `expression`]
   or_test: `and_test` | `or_test` "or" `and_test`
   and_test: `not_test` | `and_test` "and" `not_test`
   not_test: `comparison` | "not" `not_test`

In the context of Boolean operations, and also when expressions are used by
control flow statements, the following values are interpreted as false:
``False``, ``None``, numeric zero of all types, and empty strings and containers
(including strings, tuples, lists, dictionaries, sets and frozensets).  All
other values are interpreted as true.  User-defined objects can customize their
truth value by providing a :meth:`__bool__` method.

.. index:: operator: not

The operator :keyword:`not` yields ``True`` if its argument is false, ``False``
otherwise.

The expression ``x if C else y`` first evaluates *C* (*not* *x*); if *C* is
true, *x* is evaluated and its value is returned; otherwise, *y* is evaluated
and its value is returned.

.. index:: operator: and

The expression ``x and y`` first evaluates *x*; if *x* is false, its value is
returned; otherwise, *y* is evaluated and the resulting value is returned.

.. index:: operator: or

The expression ``x or y`` first evaluates *x*; if *x* is true, its value is
returned; otherwise, *y* is evaluated and the resulting value is returned.

(Note that neither :keyword:`and` nor :keyword:`or` restrict the value and type
they return to ``False`` and ``True``, but rather return the last evaluated
argument.  This is sometimes useful, e.g., if ``s`` is a string that should be
replaced by a default value if it is empty, the expression ``s or 'foo'`` yields
the desired value.  Because :keyword:`not` has to invent a value anyway, it does
not bother to return a value of the same type as its argument, so e.g., ``not
'foo'`` yields ``False``, not ``''``.)


.. _lambdas:

Lambdas
=======

.. index::
   pair: lambda; expression
   pair: lambda; form
   pair: anonymous; function

.. productionlist::
   lambda_form: "lambda" [`parameter_list`]: `expression`
   lambda_form_nocond: "lambda" [`parameter_list`]: `expression_nocond`

Lambda forms (lambda expressions) have the same syntactic position as
expressions.  They are a shorthand to create anonymous functions; the expression
``lambda arguments: expression`` yields a function object.  The unnamed object
behaves like a function object defined with ::

   def <lambda>(arguments):
       return expression

See section :ref:`function` for the syntax of parameter lists.  Note that
functions created with lambda forms cannot contain statements or annotations.

.. _lambda:


.. _exprlists:

Expression lists
================

.. index:: pair: expression; list

.. productionlist::
   expression_list: `expression` ( "," `expression` )* [","]

.. index:: object: tuple

An expression list containing at least one comma yields a tuple.  The length of
the tuple is the number of expressions in the list.  The expressions are
evaluated from left to right.

.. index:: pair: trailing; comma

The trailing comma is required only to create a single tuple (a.k.a. a
*singleton*); it is optional in all other cases.  A single expression without a
trailing comma doesn't create a tuple, but rather yields the value of that
expression. (To create an empty tuple, use an empty pair of parentheses:
``()``.)


.. _evalorder:

Evaluation order
================

.. index:: pair: evaluation; order

Python evaluates expressions from left to right.  Notice that while evaluating
an assignment, the right-hand side is evaluated before the left-hand side.

In the following lines, expressions will be evaluated in the arithmetic order of
their suffixes::

   expr1, expr2, expr3, expr4
   (expr1, expr2, expr3, expr4)
   {expr1: expr2, expr3: expr4}
   expr1 + expr2 * (expr3 - expr4)
   func(expr1, expr2, *expr3, **expr4)
   expr3, expr4 = expr1, expr2


.. _operator-summary:

Summary
=======

.. index:: pair: operator; precedence

The following table summarizes the operator precedences in Python, from lowest
precedence (least binding) to highest precedence (most binding).  Operators in
the same box have the same precedence.  Unless the syntax is explicitly given,
operators are binary.  Operators in the same box group left to right (except for
comparisons, including tests, which all have the same precedence and chain from
left to right --- see section :ref:`comparisons` --- and exponentiation, which
groups from right to left).

+----------------------------------------------+-------------------------------------+
| Operator                                     | Description                         |
+==============================================+=====================================+
| :keyword:`lambda`                            | Lambda expression                   |
+----------------------------------------------+-------------------------------------+
| :keyword:`or`                                | Boolean OR                          |
+----------------------------------------------+-------------------------------------+
| :keyword:`and`                               | Boolean AND                         |
+----------------------------------------------+-------------------------------------+
| :keyword:`not` *x*                           | Boolean NOT                         |
+----------------------------------------------+-------------------------------------+
| :keyword:`in`, :keyword:`not` :keyword:`in`  | Membership tests                    |
+----------------------------------------------+-------------------------------------+
| :keyword:`is`, :keyword:`is not`             | Identity tests                      |
+----------------------------------------------+-------------------------------------+
| ``<``, ``<=``, ``>``, ``>=``, ``!=``, ``==`` | Comparisons                         |
+----------------------------------------------+-------------------------------------+
| ``|``                                        | Bitwise OR                          |
+----------------------------------------------+-------------------------------------+
| ``^``                                        | Bitwise XOR                         |
+----------------------------------------------+-------------------------------------+
| ``&``                                        | Bitwise AND                         |
+----------------------------------------------+-------------------------------------+
| ``<<``, ``>>``                               | Shifts                              |
+----------------------------------------------+-------------------------------------+
| ``+``, ``-``                                 | Addition and subtraction            |
+----------------------------------------------+-------------------------------------+
| ``*``, ``/``, ``//``, ``%``                  | Multiplication, division, remainder |
+----------------------------------------------+-------------------------------------+
| ``+x``, ``-x``                               | Positive, negative                  |
+----------------------------------------------+-------------------------------------+
| ``~x``                                       | Bitwise not                         |
+----------------------------------------------+-------------------------------------+
| ``**``                                       | Exponentiation                      |
+----------------------------------------------+-------------------------------------+
| ``x.attribute``                              | Attribute reference                 |
+----------------------------------------------+-------------------------------------+
| ``x[index]``                                 | Subscription                        |
+----------------------------------------------+-------------------------------------+
| ``x[index:index]``                           | Slicing                             |
+----------------------------------------------+-------------------------------------+
| ``f(arguments...)``                          | Function call                       |
+----------------------------------------------+-------------------------------------+
| ``(expressions...)``                         | Binding, tuple display, generator   |
|                                              | expressions                         |
+----------------------------------------------+-------------------------------------+
| ``[expressions...]``                         | List display                        |
+----------------------------------------------+-------------------------------------+
| ``{expressions...}``                         | Dictionary or set display           |
+----------------------------------------------+-------------------------------------+

.. rubric:: Footnotes

.. [#] While ``abs(x%y) < abs(y)`` is true mathematically, for floats it may not be
   true numerically due to roundoff.  For example, and assuming a platform on which
   a Python float is an IEEE 754 double-precision number, in order that ``-1e-100 %
   1e100`` have the same sign as ``1e100``, the computed result is ``-1e-100 +
   1e100``, which is numerically exactly equal to ``1e100``.  Function :func:`fmod`
   in the :mod:`math` module returns a result whose sign matches the sign of the
   first argument instead, and so returns ``-1e-100`` in this case. Which approach
   is more appropriate depends on the application.

.. [#] If x is very close to an exact integer multiple of y, it's possible for
   ``x//y`` to be one larger than ``(x-x%y)//y`` due to rounding.  In such
   cases, Python returns the latter result, in order to preserve that
   ``divmod(x,y)[0] * y + x % y`` be very close to ``x``.

.. [#] While comparisons between strings make sense at the byte level, they may
   be counter-intuitive to users.  For example, the strings ``"\u00C7"`` and
   ``"\u0327\u0043"`` compare differently, even though they both represent the
   same unicode character (LATIN CAPTITAL LETTER C WITH CEDILLA).  To compare
   strings in a human recognizable way, compare using
   :func:`unicodedata.normalize`.

.. [#] The implementation computes this efficiently, without constructing lists
   or sorting.

.. [#] Earlier versions of Python used lexicographic comparison of the sorted (key,
   value) lists, but this was very expensive for the common case of comparing
   for equality.  An even earlier version of Python compared dictionaries by
   identity only, but this caused surprises because people expected to be able
   to test a dictionary for emptiness by comparing it to ``{}``.