8.13. enum
— Support for enumerations¶
Nuevo en la versión 3.4.
Source code: Lib/enum.py
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.
8.13.1. 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()
.
-
class
enum.
Enum
¶ Base class for creating enumerated constants. See section Functional API for an alternate construction syntax.
-
enum.
unique
()¶ Enum class decorator that ensures only one name is bound to any one value.
8.13.2. 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
...
Nota
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
isred
, the value ofColor.blue
is3
, etc.)
Nota
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 definition order:
>>> class Shake(Enum):
... vanilla = 7
... chocolate = 4
... cookies = 9
... mint = 3
...
>>> for shake in Shake:
... print(shake)
...
Shake.vanilla
Shake.chocolate
Shake.cookies
Shake.mint
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
8.13.3. 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 you have an enum member and need its name
or value
:
>>> member = Color.red
>>> member.name
'red'
>>> member.value
1
8.13.4. Duplicating enum members and values¶
Having two enum members with the same name is invalid:
>>> 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):
... 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>
Nota
Attempting to create a member with the same name as an already defined attribute (another member, a method, etc.) or attempting to create an attribute with the same name as a member is not allowed.
8.13.5. Ensuring unique enumeration values¶
By default, enumerations allow multiple names as aliases for the same value. When this behavior isn’t desired, the following decorator can be used to ensure each value is used only once in the enumeration:
-
@
enum.
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:
>>> from enum import Enum, unique
>>> @unique
... class Mistake(Enum):
... one = 1
... two = 2
... three = 3
... four = 3
...
Traceback (most recent call last):
...
ValueError: duplicate values found in <enum 'Mistake'>: four -> three
8.13.6. Iteration¶
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 an ordered dictionary mapping names
to members. It includes all names defined in the enumeration, including the
aliases:
>>> for name, member in Shape.__members__.items():
... name, member
...
('square', <Shape.square: 2>)
('diamond', <Shape.diamond: 1>)
('circle', <Shape.circle: 3>)
('alias_for_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']
8.13.7. 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()
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
8.13.8. 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: names that start and end 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 special methods (__str__()
,
__add__()
, etc.) 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.
8.13.9. 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.)
8.13.10. Pickling¶
Enumerations can be pickled and unpickled:
>>> from test.test_enum import Fruit
>>> from pickle import dumps, loads
>>> Fruit.tomato is loads(dumps(Fruit.tomato))
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.
Nota
With pickle protocol version 4 it is possible to easily pickle enums nested in other classes.
It is possible to modify how Enum members are pickled/unpickled by defining
__reduce_ex__()
in the enumeration class.
8.13.11. 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 (use
the start
parameter to specify a different starting value). A
new class derived from Enum
is returned. In other words, the above
assignment to Animal
is equivalent to:
>>> class Animal(Enum):
... ant = 1
... bee = 2
... cat = 3
... dog = 4
...
The reason for defaulting to 1
as the starting number and not 0
is
that 0
is False
in a boolean sense, but enum members all evaluate
to True
.
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:
>>> Animal = Enum('Animal', 'ant bee cat dog', module=__name__)
Advertencia
If module
is not supplied, and Enum cannot determine what it is,
the new Enum members will not be unpicklable; to keep errors closer to
the source, pickling will be disabled.
The new pickle protocol 4 also, in some circumstances, relies on
__qualname__
being set to the location where pickle will be able
to find the class. For example, if the class was made available in class
SomeData in the global scope:
>>> Animal = Enum('Animal', 'ant bee cat dog', qualname='SomeData.Animal')
The complete signature is:
Enum(value='NewEnumName', names=<...>, *, module='...', qualname='...', type=<mixed-in class>, start=1)
value: | What the new Enum class will record as its name. |
---|---|
names: | The Enum members. This can be a whitespace or comma separated string (values will start at 1 unless otherwise specified): 'red green blue' | 'red,green,blue' | 'red, green, blue'
or an iterator of names: ['red', 'green', 'blue']
or an iterator of (name, value) pairs: [('cyan', 4), ('magenta', 5), ('yellow', 6)]
or a mapping: {'chartreuse': 7, 'sea_green': 11, 'rosemary': 42}
|
module: | name of module where new Enum class can be found. |
qualname: | where in module new Enum class can be found. |
type: | type to mix in to new Enum class. |
start: | number to start counting at if only names are passed in. |
Distinto en la versión 3.5: The start parameter was added.
8.13.12. Derived Enumerations¶
8.13.12.1. 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.
8.13.12.2. 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:
- When subclassing
Enum
, mix-in types must appear beforeEnum
itself in the sequence of bases, as in theIntEnum
example above. - 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 asint
orstr
. - When another data type is mixed in, the
value
attribute is not the same as the enum member itself, although it is equivalent and will compare equal. - %-style formatting: %s and %r call the
Enum
class’s__str__()
and__repr__()
respectively; other codes (such as %i or %h for IntEnum) treat the enum member as its mixed-in type. str.format()
(orformat()
) will use the mixed-in type’s__format__()
. If theEnum
class’sstr()
orrepr()
is desired, use the !s or !r format codes.
8.13.13. 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.
8.13.13.1. 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):
... red = ()
... green = ()
... blue = ()
...
>>> Color.green.value == 2
True
8.13.13.2. 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):
... A = 5
... B = 4
... C = 3
... D = 2
... F = 1
...
>>> Grade.C < Grade.A
True
8.13.13.3. DuplicateFreeEnum¶
Raises an error if a duplicate member name is found instead of creating an alias:
>>> class DuplicateFreeEnum(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 DuplicateFreeEnum: %r --> %r"
... % (a, e))
...
>>> class Color(DuplicateFreeEnum):
... red = 1
... green = 2
... blue = 3
... grene = 2
...
Traceback (most recent call last):
...
ValueError: aliases not allowed in DuplicateFreeEnum: 'grene' --> 'green'
Nota
This is a useful example for subclassing Enum to add or change other
behaviors as well as disallowing aliases. If the only desired change is
disallowing aliases, the unique()
decorator can be used instead.
8.13.13.4. 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
8.13.14. How are Enums different?¶
Enums have a custom metaclass that affects many aspects of both derived Enum classes and their instances (members).
8.13.14.1. 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__()
).
8.13.14.2. 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.
8.13.14.3. Finer Points¶
Enum
members are instances of an Enum
class, and even
though they are accessible as EnumClass.member, they should not be accessed
directly from the member as that lookup may fail or, worse, return something
besides the Enum
member you looking for:
>>> class FieldTypes(Enum):
... name = 0
... value = 1
... size = 2
...
>>> FieldTypes.value.size
<FieldTypes.size: 2>
>>> FieldTypes.size.value
2
Distinto en la versión 3.5.
The __members__
attribute is only available on the class.
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']
The __new__()
method will only be used for the creation of the
Enum
members – after that it is replaced. Any custom __new__()
method must create the object and set the _value_
attribute
appropriately.
If you wish to change how Enum
members are looked up you should either
write a helper function or a classmethod()
for the Enum
subclass.