path: root/enum/doc/enum.rst

``enum`` --- support for enumerations
========================================

.. :synopsis: enumerations are sets of symbolic names bound to unique, constant
  values.
.. :moduleauthor:: Ethan Furman <ethan@stoneleaf.us>
.. :sectionauthor:: Barry Warsaw <barry@python.org>,
.. :sectionauthor:: Eli Bendersky <eliben@gmail.com>,
.. :sectionauthor:: Ethan Furman <ethan@stoneleaf.us>

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

An enumeration is a set of symbolic names (members) bound to unique, constant
values.  Within an enumeration, the members can be compared by identity, and
the enumeration itself can be iterated over.


Module Contents
---------------

This module defines two enumeration classes that can be used to define unique
sets of names and values: ``Enum`` and ``IntEnum``.  It also defines
one decorator, ``unique``.

``Enum``

Base class for creating enumerated constants.  See section `Functional API`_
for an alternate construction syntax.

``IntEnum``

Base class for creating enumerated constants that are also subclasses of ``int``.

``unique``

Enum class decorator that ensures only one name is bound to any one value.


Creating an Enum
----------------

Enumerations are created using the ``class`` syntax, which makes them
easy to read and write.  An alternative creation method is described in
`Functional API`_.  To define an enumeration, subclass ``Enum`` as
follows::

    >>> from enum import Enum
    >>> class Color(Enum):
    ...     red = 1
    ...     green = 2
    ...     blue = 3

Note: Nomenclature

  - The class ``Color`` is an *enumeration* (or *enum*)
  - The attributes ``Color.red``, ``Color.green``, etc., are
    *enumeration members* (or *enum members*).
  - The enum members have *names* and *values* (the name of
    ``Color.red`` is ``red``, the value of ``Color.blue`` is
    ``3``, etc.)
    
Note:

    Even though we use the ``class`` syntax to create Enums, Enums
    are not normal Python classes.  See `How are Enums different?`_ for
    more details.

Enumeration members have human readable string representations::

    >>> print(Color.red)
    Color.red

...while their ``repr`` has more information::

    >>> print(repr(Color.red))
    <Color.red: 1>

The *type* of an enumeration member is the enumeration it belongs to::

    >>> type(Color.red)
    <enum 'Color'>
    >>> isinstance(Color.green, Color)
    True
    >>>

Enum members also have a property that contains just their item name::

    >>> print(Color.red.name)
    red

Enumerations support iteration.  In Python 3.x definition order is used; in
Python 2.x the definition order is not available, but class attribute
``__order__`` is supported;  otherwise, value order is used::

    >>> class Shake(Enum):
    ...   __order__ = 'vanilla chocolate cookies mint'  # only needed in 2.x
    ...   vanilla = 7
    ...   chocolate = 4
    ...   cookies = 9
    ...   mint = 3
    ...
    >>> for shake in Shake:
    ...   print(shake)
    ...
    Shake.vanilla
    Shake.chocolate
    Shake.cookies
    Shake.mint

The ``__order__`` attribute is always removed, and in 3.x it is also ignored
(order is definition order); however, in the stdlib version it will be ignored
but not removed.

Enumeration members are hashable, so they can be used in dictionaries and sets::

    >>> apples = {}
    >>> apples[Color.red] = 'red delicious'
    >>> apples[Color.green] = 'granny smith'
    >>> apples == {Color.red: 'red delicious', Color.green: 'granny smith'}
    True


Programmatic access to enumeration members and their attributes
---------------------------------------------------------------

Sometimes it's useful to access members in enumerations programmatically (i.e.
situations where ``Color.red`` won't do because the exact color is not known
at program-writing time).  ``Enum`` allows such access::

    >>> Color(1)
    <Color.red: 1>
    >>> Color(3)
    <Color.blue: 3>

If you want to access enum members by *name*, use item access::

    >>> Color['red']
    <Color.red: 1>
    >>> Color['green']
    <Color.green: 2>

If have an enum member and need its ``name`` or ``value``::

    >>> member = Color.red
    >>> member.name
    'red'
    >>> member.value
    1


Duplicating enum members and values
-----------------------------------

Having two enum members (or any other attribute) with the same name is invalid;
in Python 3.x this would raise an error, but in Python 2.x the second member
simply overwrites the first::

    >>> # python 2.x
    >>> class Shape(Enum):
    ...   square = 2
    ...   square = 3
    ...
    >>> Shape.square
    <Shape.square: 3>

    >>> # python 3.x
    >>> class Shape(Enum):
    ...   square = 2
    ...   square = 3
    Traceback (most recent call last):
    ...
    TypeError: Attempted to reuse key: 'square'

However, two enum members are allowed to have the same value.  Given two members
A and B with the same value (and A defined first), B is an alias to A.  By-value
lookup of the value of A and B will return A.  By-name lookup of B will also
return A::

    >>> class Shape(Enum):
    ...   __order__ = 'square diamond circle alias_for_square'  # only needed in 2.x
    ...   square = 2
    ...   diamond = 1
    ...   circle = 3
    ...   alias_for_square = 2
    ...
    >>> Shape.square
    <Shape.square: 2>
    >>> Shape.alias_for_square
    <Shape.square: 2>
    >>> Shape(2)
    <Shape.square: 2>


Allowing aliases is not always desirable.  ``unique`` can be used to ensure
that none exist in a particular enumeration::

    >>> from enum import unique
    >>> @unique
    ... class Mistake(Enum):
    ...   __order__ = 'one two three four'  # only needed in 2.x
    ...   one = 1
    ...   two = 2
    ...   three = 3
    ...   four = 3
    Traceback (most recent call last):
    ...
    ValueError: duplicate names found in <enum 'Mistake'>: four -> three

Iterating over the members of an enum does not provide the aliases::

    >>> list(Shape)
    [<Shape.square: 2>, <Shape.diamond: 1>, <Shape.circle: 3>]

The special attribute ``__members__`` is a dictionary mapping names to members.
It includes all names defined in the enumeration, including the aliases::

    >>> for name, member in sorted(Shape.__members__.items()):
    ...   name, member
    ...
    ('alias_for_square', <Shape.square: 2>)
    ('circle', <Shape.circle: 3>)
    ('diamond', <Shape.diamond: 1>)
    ('square', <Shape.square: 2>)

The ``__members__`` attribute can be used for detailed programmatic access to
the enumeration members.  For example, finding all the aliases::

    >>> [name for name, member in Shape.__members__.items() if member.name != name]
    ['alias_for_square']

Comparisons
-----------

Enumeration members are compared by identity::

    >>> Color.red is Color.red
    True
    >>> Color.red is Color.blue
    False
    >>> Color.red is not Color.blue
    True

Ordered comparisons between enumeration values are *not* supported.  Enum
members are not integers (but see `IntEnum`_ below)::

    >>> Color.red < Color.blue
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: unorderable types: Color() < Color()

.. warning::

    In Python 2 *everything* is ordered, even though the ordering may not
    make sense.  If you want your enumerations to have a sensible ordering
    check out the `OrderedEnum`_ recipe below.


Equality comparisons are defined though::

    >>> Color.blue == Color.red
    False
    >>> Color.blue != Color.red
    True
    >>> Color.blue == Color.blue
    True

Comparisons against non-enumeration values will always compare not equal
(again, ``IntEnum`` was explicitly designed to behave differently, see
below)::

    >>> Color.blue == 2
    False


Allowed members and attributes of enumerations
----------------------------------------------

The examples above use integers for enumeration values.  Using integers is
short and handy (and provided by default by the `Functional API`_), but not
strictly enforced.  In the vast majority of use-cases, one doesn't care what
the actual value of an enumeration is.  But if the value *is* important,
enumerations can have arbitrary values.

Enumerations are Python classes, and can have methods and special methods as
usual.  If we have this enumeration::

    >>> class Mood(Enum):
    ...   funky = 1
    ...   happy = 3
    ... 
    ...   def describe(self):
    ...     # self is the member here
    ...     return self.name, self.value
    ... 
    ...   def __str__(self):
    ...     return 'my custom str! {0}'.format(self.value)
    ... 
    ...   @classmethod
    ...   def favorite_mood(cls):
    ...     # cls here is the enumeration
    ...     return cls.happy

Then::

    >>> Mood.favorite_mood()
    <Mood.happy: 3>
    >>> Mood.happy.describe()
    ('happy', 3)
    >>> str(Mood.funky)
    'my custom str! 1'

The rules for what is allowed are as follows: _sunder_ names (starting and
ending with a single underscore) are reserved by enum and cannot be used;
all other attributes defined within an enumeration will become members of this
enumeration, with the exception of *__dunder__* names and descriptors (methods
are also descriptors).

Note:

    If your enumeration defines ``__new__`` and/or ``__init__`` then
    whatever value(s) were given to the enum member will be passed into
    those methods.  See `Planet`_ for an example.


Restricted subclassing of enumerations
--------------------------------------

Subclassing an enumeration is allowed only if the enumeration does not define
any members.  So this is forbidden::

    >>> class MoreColor(Color):
    ...   pink = 17
    Traceback (most recent call last):
    ...
    TypeError: Cannot extend enumerations

But this is allowed::

    >>> class Foo(Enum):
    ...   def some_behavior(self):
    ...     pass
    ...
    >>> class Bar(Foo):
    ...   happy = 1
    ...   sad = 2
    ...

Allowing subclassing of enums that define members would lead to a violation of
some important invariants of types and instances.  On the other hand, it makes
sense to allow sharing some common behavior between a group of enumerations.
(See `OrderedEnum`_ for an example.)


Pickling
--------

Enumerations can be pickled and unpickled::

    >>> from enum.test_enum import Fruit
    >>> from pickle import dumps, loads
    >>> Fruit.tomato is loads(dumps(Fruit.tomato, 2))
    True

The usual restrictions for pickling apply: picklable enums must be defined in
the top level of a module, since unpickling requires them to be importable
from that module.

Note:

    With pickle protocol version 4 (introduced in Python 3.4) it is possible
    to easily pickle enums nested in other classes.



Functional API
--------------

The ``Enum`` class is callable, providing the following functional API::

    >>> Animal = Enum('Animal', 'ant bee cat dog')
    >>> Animal
    <enum 'Animal'>
    >>> Animal.ant
    <Animal.ant: 1>
    >>> Animal.ant.value
    1
    >>> list(Animal)
    [<Animal.ant: 1>, <Animal.bee: 2>, <Animal.cat: 3>, <Animal.dog: 4>]

The semantics of this API resemble ``namedtuple``. The first argument
of the call to ``Enum`` is the name of the enumeration. 

The second argument is the *source* of enumeration member names.  It can be a
whitespace-separated string of names, a sequence of names, a sequence of
2-tuples with key/value pairs, or a mapping (e.g. dictionary) of names to
values.  The last two options enable assigning arbitrary values to
enumerations; the others auto-assign increasing integers starting with 1.  A
new class derived from ``Enum`` is returned.  In other words, the above
assignment to ``Animal`` is equivalent to::

    >>> class Animals(Enum):
    ...   ant = 1
    ...   bee = 2
    ...   cat = 3
    ...   dog = 4

Pickling enums created with the functional API can be tricky as frame stack
implementation details are used to try and figure out which module the
enumeration is being created in (e.g. it will fail if you use a utility
function in separate module, and also may not work on IronPython or Jython).
The solution is to specify the module name explicitly as follows::

    >>> Animals = Enum('Animals', 'ant bee cat dog', module=__name__)

Derived Enumerations
--------------------

IntEnum
^^^^^^^

A variation of ``Enum`` is provided which is also a subclass of
``int``.  Members of an ``IntEnum`` can be compared to integers;
by extension, integer enumerations of different types can also be compared
to each other::

    >>> from enum import IntEnum
    >>> class Shape(IntEnum):
    ...   circle = 1
    ...   square = 2
    ...
    >>> class Request(IntEnum):
    ...   post = 1
    ...   get = 2
    ...
    >>> Shape == 1
    False
    >>> Shape.circle == 1
    True
    >>> Shape.circle == Request.post
    True

However, they still can't be compared to standard ``Enum`` enumerations::

    >>> class Shape(IntEnum):
    ...   circle = 1
    ...   square = 2
    ...
    >>> class Color(Enum):
    ...   red = 1
    ...   green = 2
    ...
    >>> Shape.circle == Color.red
    False

``IntEnum`` values behave like integers in other ways you'd expect::

    >>> int(Shape.circle)
    1
    >>> ['a', 'b', 'c'][Shape.circle]
    'b'
    >>> [i for i in range(Shape.square)]
    [0, 1]

For the vast majority of code, ``Enum`` is strongly recommended,
since ``IntEnum`` breaks some semantic promises of an enumeration (by
being comparable to integers, and thus by transitivity to other
unrelated enumerations).  It should be used only in special cases where
there's no other choice; for example, when integer constants are
replaced with enumerations and backwards compatibility is required with code
that still expects integers.


Others
^^^^^^

While ``IntEnum`` is part of the ``enum`` module, it would be very
simple to implement independently::

    class IntEnum(int, Enum):
        pass

This demonstrates how similar derived enumerations can be defined; for example
a ``StrEnum`` that mixes in ``str`` instead of ``int``.

Some rules:

1. When subclassing ``Enum``, mix-in types must appear before
   ``Enum`` itself in the sequence of bases, as in the ``IntEnum``
   example above.
2. While ``Enum`` can have members of any type, once you mix in an
   additional type, all the members must have values of that type, e.g.
   ``int`` above.  This restriction does not apply to mix-ins which only
   add methods and don't specify another data type such as ``int`` or
   ``str``.
3. When another data type is mixed in, the ``value`` attribute is *not the
   same* as the enum member itself, although it is equivalant and will compare
   equal.
4. %-style formatting:  ``%s`` and ``%r`` call ``Enum``'s ``__str__`` and
   ``__repr__`` respectively; other codes (such as ``%i`` or ``%h`` for
   IntEnum) treat the enum member as its mixed-in type.

   Note: Prior to Python 3.4 there is a bug in ``str``'s %-formatting: ``int``
   subclasses are printed as strings and not numbers when the ``%d``, ``%i``,
   or ``%u`` codes are used.
5. ``str.__format__`` (or ``format``) will use the mixed-in
   type's ``__format__``.  If the ``Enum``'s ``str`` or
   ``repr`` is desired use the ``!s`` or ``!r`` ``str`` format codes.


Decorators
----------

unique
^^^^^^

A ``class`` decorator specifically for enumerations.  It searches an
enumeration's ``__members__`` gathering any aliases it finds; if any are
found ``ValueError`` is raised with the details::

    >>> @unique
    ... class NoDupes(Enum):
    ...    first = 'one'
    ...    second = 'two'
    ...    third = 'two'
    Traceback (most recent call last):
    ...
    ValueError: duplicate names found in <enum 'NoDupes'>: third -> second


Interesting examples
--------------------

While ``Enum`` and ``IntEnum`` are expected to cover the majority of
use-cases, they cannot cover them all.  Here are recipes for some different
types of enumerations that can be used directly, or as examples for creating
one's own.


AutoNumber
^^^^^^^^^^

Avoids having to specify the value for each enumeration member::

    >>> class AutoNumber(Enum):
    ...     def __new__(cls):
    ...         value = len(cls.__members__) + 1
    ...         obj = object.__new__(cls)
    ...         obj._value_ = value
    ...         return obj
    ...
    >>> class Color(AutoNumber):
    ...     __order__ = "red green blue"  # only needed in 2.x
    ...     red = ()
    ...     green = ()
    ...     blue = ()
    ...
    >>> Color.green.value == 2
    True

Note:

    The `__new__` method, if defined, is used during creation of the Enum
    members; it is then replaced by Enum's `__new__` which is used after
    class creation for lookup of existing members.  Due to the way Enums are
    supposed to behave, there is no way to customize Enum's `__new__`.


UniqueEnum
^^^^^^^^^^

Raises an error if a duplicate member name is found instead of creating an
alias::

    >>> class UniqueEnum(Enum):
    ...     def __init__(self, *args):
    ...         cls = self.__class__
    ...         if any(self.value == e.value for e in cls):
    ...             a = self.name
    ...             e = cls(self.value).name
    ...             raise ValueError(
    ...                     "aliases not allowed in UniqueEnum:  %r --> %r"
    ...                     % (a, e))
    ... 
    >>> class Color(UniqueEnum):
    ...     red = 1
    ...     green = 2
    ...     blue = 3
    ...     grene = 2
    Traceback (most recent call last):
    ...
    ValueError: aliases not allowed in UniqueEnum:  'grene' --> 'green'
    

OrderedEnum
^^^^^^^^^^^

An ordered enumeration that is not based on ``IntEnum`` and so maintains
the normal ``Enum`` invariants (such as not being comparable to other
enumerations)::

    >>> class OrderedEnum(Enum):
    ...     def __ge__(self, other):
    ...         if self.__class__ is other.__class__:
    ...             return self._value_ >= other._value_
    ...         return NotImplemented
    ...     def __gt__(self, other):
    ...         if self.__class__ is other.__class__:
    ...             return self._value_ > other._value_
    ...         return NotImplemented
    ...     def __le__(self, other):
    ...         if self.__class__ is other.__class__:
    ...             return self._value_ <= other._value_
    ...         return NotImplemented
    ...     def __lt__(self, other):
    ...         if self.__class__ is other.__class__:
    ...             return self._value_ < other._value_
    ...         return NotImplemented
    ...
    >>> class Grade(OrderedEnum):
    ...     __ordered__ = 'A B C D F'
    ...     A = 5
    ...     B = 4
    ...     C = 3
    ...     D = 2
    ...     F = 1
    ...
    >>> Grade.C < Grade.A
    True


Planet
^^^^^^

If ``__new__`` or ``__init__`` is defined the value of the enum member
will be passed to those methods::

    >>> class Planet(Enum):
    ...     MERCURY = (3.303e+23, 2.4397e6)
    ...     VENUS   = (4.869e+24, 6.0518e6)
    ...     EARTH   = (5.976e+24, 6.37814e6)
    ...     MARS    = (6.421e+23, 3.3972e6)
    ...     JUPITER = (1.9e+27,   7.1492e7)
    ...     SATURN  = (5.688e+26, 6.0268e7)
    ...     URANUS  = (8.686e+25, 2.5559e7)
    ...     NEPTUNE = (1.024e+26, 2.4746e7)
    ...     def __init__(self, mass, radius):
    ...         self.mass = mass       # in kilograms
    ...         self.radius = radius   # in meters
    ...     @property
    ...     def surface_gravity(self):
    ...         # universal gravitational constant  (m3 kg-1 s-2)
    ...         G = 6.67300E-11
    ...         return G * self.mass / (self.radius * self.radius)
    ... 
    >>> Planet.EARTH.value
    (5.976e+24, 6378140.0)
    >>> Planet.EARTH.surface_gravity
    9.802652743337129


How are Enums different?
------------------------

Enums have a custom metaclass that affects many aspects of both derived Enum
classes and their instances (members).


Enum Classes
^^^^^^^^^^^^

The ``EnumMeta`` metaclass is responsible for providing the
``__contains__``, ``__dir__``, ``__iter__`` and other methods that
allow one to do things with an ``Enum`` class that fail on a typical
class, such as ``list(Color)`` or ``some_var in Color``.  ``EnumMeta`` is
responsible for ensuring that various other methods on the final ``Enum``
class are correct (such as ``__new__``, ``__getnewargs__``,
``__str__`` and ``__repr__``)


Enum Members (aka instances)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The most interesting thing about Enum members is that they are singletons.
``EnumMeta`` creates them all while it is creating the ``Enum``
class itself, and then puts a custom ``__new__`` in place to ensure
that no new ones are ever instantiated by returning only the existing
member instances.


Finer Points
^^^^^^^^^^^^

Enum members are instances of an Enum class, and even though they are
accessible as ``EnumClass.member``, they are not accessible directly from
the member::

    >>> Color.red
    <Color.red: 1>
    >>> Color.red.blue
    Traceback (most recent call last):
    ...
    AttributeError: 'Color' object has no attribute 'blue'

Likewise, ``__members__`` is only available on the class.

In Python 3.x ``__members__`` is always an ``OrderedDict``, with the order being
the definition order.  In Python 2.7 ``__members__`` is an ``OrderedDict`` if
``__order__`` was specified, and a plain ``dict`` otherwise.  In all other Python
2.x versions ``__members__`` is a plain ``dict`` even if ``__order__`` was specified
as the ``OrderedDict`` type didn't exist yet.

If you give your ``Enum`` subclass extra methods, like the `Planet`_
class above, those methods will show up in a `dir` of the member,
but not of the class::

    >>> dir(Planet)
    ['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS',
    'VENUS', '__class__', '__doc__', '__members__', '__module__']
    >>> dir(Planet.EARTH)
    ['__class__', '__doc__', '__module__', 'name', 'surface_gravity', 'value']

A ``__new__`` method will only be used for the creation of the
``Enum`` members -- after that it is replaced.  This means if you wish to
change how ``Enum`` members are looked up you either have to write a
helper function or a ``classmethod``.