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Python 3.4 Enum backported to 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4

Project description

``enum`` --- support for enumerations

.. :synopsis: enumerations are sets of symbolic names bound to unique, constant
.. :moduleauthor:: Ethan Furman <>
.. :sectionauthor:: Barry Warsaw <>,
.. :sectionauthor:: Eli Bendersky <>,
.. :sectionauthor:: Ethan Furman <>


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.

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``, that ensures only unique member names are present
in an enumeration.

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

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

**A note on nomenclature**: we call ``Color`` an *enumeration* (or *enum*)
and ````, ```` are *enumeration members* (or
*enum members*). Enumeration members also have *values* (the value of
```` is ``1``, etc.)

Enumeration members have human readable string representations::

>>> print(

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

>>> print(repr(
< 1>

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

>>> type(
<enum 'Color'>
>>> isinstance(, Color)

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

>>> print(

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'
... vanilla = 7
... chocolate = 4
... cookies = 9
... mint = 3
>>> for shake in Shake:
... print(shake)

The ``__order__`` attribute is ignored, but still removed, in 3.x; 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[] = 'red delicious'
>>> apples[] = 'granny smith'
>>> apples == { 'red delicious', 'granny smith'}

Programmatic access to enumeration members and their attributes

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

>>> Color(1)
< 1>
>>> Color(3)
< 3>

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

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

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

>>> member =
>>> member.value

Duplicating enum members and values

Having two enum members 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::

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

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'
... 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'
... 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>, < 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', < 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 != name]


Enumeration members are compared by identity::

>>> is
>>> is
>>> is not

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

>>> <
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::

>>> ==
>>> !=
>>> ==

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

>>> == 2

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.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


>>> 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.)


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))

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.

.. warning::

In order to support the singleton nature of enumeration members, pickle
protocol version 2 or higher must be used. The default in Python 2.x is 0.

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
>>> list(Animal)
[<Animal.ant: 1>, <Animal.bee: 2>, < 3>, < 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


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
>>> == 1
>>> ==

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
>>> ==

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

>>> int(
>>> ['a', 'b', 'c'][]
>>> [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.


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

class IntEnum(int, Enum):

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
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



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.


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):
... red = ()
... green = ()
... blue = ()
>>> == 2


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

>>> class UniqueEnum(Enum):
... def __init__(self, *args):
... cls = self.__class__
... if any(self.value == e.value for e in cls):
... a =
... 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'


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

>>> 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


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

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