enum
— Support for enumerations¶
버전 3.4에 추가.
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.
참고
Case of Enum Members
Because Enums are used to represent constants we recommend using UPPER_CASE names for enum members, and will be using that style in our examples.
Module Contents¶
This module defines four enumeration classes that can be used to define unique
sets of names and values: Enum
, IntEnum
, Flag
, and
IntFlag
. It also defines one decorator, unique()
, and one
helper, auto
.
-
class
enum.
Enum
¶ Base class for creating enumerated constants. See section Functional API for an alternate construction syntax.
-
class
enum.
IntFlag
¶ Base class for creating enumerated constants that can be combined using the bitwise operators without losing their
IntFlag
membership.IntFlag
members are also subclasses ofint
.
-
class
enum.
Flag
¶ Base class for creating enumerated constants that can be combined using the bitwise operations without losing their
Flag
membership.
-
enum.
unique
() Enum class decorator that ensures only one name is bound to any one value.
-
class
enum.
auto
¶ Instances are replaced with an appropriate value for Enum members. Initial value starts at 1.
버전 3.6에 추가: Flag
, IntFlag
, auto
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
...
참고
Enum member values
Member values can be anything: int
, str
, etc.. If
the exact value is unimportant you may use auto
instances and an
appropriate value will be chosen for you. Care must be taken if you mix
auto
with other values.
참고
Nomenclature
The class
Color
is an enumeration (or enum)The attributes
Color.RED
,Color.GREEN
, etc., are enumeration members (or enum members) and are functionally constants.The enum members have names and values (the name of
Color.RED
isRED
, the value ofColor.BLUE
is3
, etc.)
참고
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
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
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>
참고
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.
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
Using automatic values¶
If the exact value is unimportant you can use auto
:
>>> from enum import Enum, auto
>>> class Color(Enum):
... RED = auto()
... BLUE = auto()
... GREEN = auto()
...
>>> list(Color)
[<Color.RED: 1>, <Color.BLUE: 2>, <Color.GREEN: 3>]
The values are chosen by _generate_next_value_()
, which can be
overridden:
>>> class AutoName(Enum):
... def _generate_next_value_(name, start, count, last_values):
... return name
...
>>> class Ordinal(AutoName):
... NORTH = auto()
... SOUTH = auto()
... EAST = auto()
... WEST = auto()
...
>>> list(Ordinal)
[<Ordinal.NORTH: 'NORTH'>, <Ordinal.SOUTH: 'SOUTH'>, <Ordinal.EAST: 'EAST'>, <Ordinal.WEST: 'WEST'>]
참고
The goal of the default _generate_next_value_()
method is to provide
the next int
in sequence with the last int
provided, but
the way it does this is an implementation detail and may change.
참고
The _generate_next_value_()
method must be defined before any members.
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 a read-only ordered mapping of 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']
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: '<' not supported between instances of 'Color' and '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
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.), descriptors (methods are also descriptors), and
variable names listed in _ignore_
.
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 Enum subclassing¶
A new Enum
class must have one base Enum class, up to one concrete
data type, and as many object
-based mixin classes as needed. The
order of these base classes is:
class EnumName([mix-in, ...,] [data-type,] base-enum):
pass
Also, 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 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.
참고
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.
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__)
경고
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.
버전 3.5에서 변경: The start parameter was added.
Derived Enumerations¶
IntEnum¶
The first variation of Enum
that is provided 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]
IntFlag¶
The next variation of Enum
provided, IntFlag
, is also based
on int
. The difference being IntFlag
members can be combined
using the bitwise operators (&, |, ^, ~) and the result is still an
IntFlag
member. However, as the name implies, IntFlag
members also subclass int
and can be used wherever an int
is
used. Any operation on an IntFlag
member besides the bit-wise
operations will lose the IntFlag
membership.
버전 3.6에 추가.
Sample IntFlag
class:
>>> from enum import IntFlag
>>> class Perm(IntFlag):
... R = 4
... W = 2
... X = 1
...
>>> Perm.R | Perm.W
<Perm.R|W: 6>
>>> Perm.R + Perm.W
6
>>> RW = Perm.R | Perm.W
>>> Perm.R in RW
True
It is also possible to name the combinations:
>>> class Perm(IntFlag):
... R = 4
... W = 2
... X = 1
... RWX = 7
>>> Perm.RWX
<Perm.RWX: 7>
>>> ~Perm.RWX
<Perm.-8: -8>
Another important difference between IntFlag
and Enum
is that
if no flags are set (the value is 0), its boolean evaluation is False
:
>>> Perm.R & Perm.X
<Perm.0: 0>
>>> bool(Perm.R & Perm.X)
False
Because IntFlag
members are also subclasses of int
they can
be combined with them:
>>> Perm.X | 8
<Perm.8|X: 9>
Flag¶
The last variation is Flag
. Like IntFlag
, Flag
members can be combined using the bitwise operators (&, |, ^, ~). Unlike
IntFlag
, they cannot be combined with, nor compared against, any
other Flag
enumeration, nor int
. While it is possible to
specify the values directly it is recommended to use auto
as the
value and let Flag
select an appropriate value.
버전 3.6에 추가.
Like IntFlag
, if a combination of Flag
members results in no
flags being set, the boolean evaluation is False
:
>>> from enum import Flag, auto
>>> class Color(Flag):
... RED = auto()
... BLUE = auto()
... GREEN = auto()
...
>>> Color.RED & Color.GREEN
<Color.0: 0>
>>> bool(Color.RED & Color.GREEN)
False
Individual flags should have values that are powers of two (1, 2, 4, 8, …), while combinations of flags won’t:
>>> class Color(Flag):
... RED = auto()
... BLUE = auto()
... GREEN = auto()
... WHITE = RED | BLUE | GREEN
...
>>> Color.WHITE
<Color.WHITE: 7>
Giving a name to the “no flags set” condition does not change its boolean value:
>>> class Color(Flag):
... BLACK = 0
... RED = auto()
... BLUE = auto()
... GREEN = auto()
...
>>> Color.BLACK
<Color.BLACK: 0>
>>> bool(Color.BLACK)
False
참고
For the majority of new code, Enum
and Flag
are strongly
recommended, since IntEnum
and IntFlag
break some
semantic promises of an enumeration (by being comparable to integers, and
thus by transitivity to other unrelated enumerations). IntEnum
and IntFlag
should be used only in cases where Enum
and
Flag
will not do; for example, when integer constants are replaced
with enumerations, or for interoperability with other systems.
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.Formatted string literals,
str.format()
, andformat()
will use the mixed-in type’s__format__()
unless__str__()
or__format__()
is overridden in the subclass, in which case the overridden methods orEnum
methods will be used. Use the !s and !r format codes to force usage of theEnum
class’s__str__()
and__repr__()
methods.
When to use __new__()
vs. __init__()
¶
__new__()
must be used whenever you want to customize the actual value of
the Enum
member. Any other modifications may go in either
__new__()
or __init__()
, with __init__()
being preferred.
For example, if you want to pass several items to the constructor, but only want one of them to be the value:
>>> class Coordinate(bytes, Enum):
... """
... Coordinate with binary codes that can be indexed by the int code.
... """
... def __new__(cls, value, label, unit):
... obj = bytes.__new__(cls, [value])
... obj._value_ = value
... obj.label = label
... obj.unit = unit
... return obj
... PX = (0, 'P.X', 'km')
... PY = (1, 'P.Y', 'km')
... VX = (2, 'V.X', 'km/s')
... VY = (3, 'V.Y', 'km/s')
...
>>> print(Coordinate['PY'])
Coordinate.PY
>>> print(Coordinate(3))
Coordinate.VY
Interesting examples¶
While Enum
, IntEnum
, IntFlag
, and Flag
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.
Omitting values¶
In many use-cases one doesn’t care what the actual value of an enumeration is. There are several ways to define this type of simple enumeration:
use instances of
auto
for the valueuse instances of
object
as the valueuse a descriptive string as the value
use a tuple as the value and a custom
__new__()
to replace the tuple with anint
value
Using any of these methods signifies to the user that these values are not important, and also enables one to add, remove, or reorder members without having to renumber the remaining members.
Whichever method you choose, you should provide a repr()
that also hides
the (unimportant) value:
>>> class NoValue(Enum):
... def __repr__(self):
... return '<%s.%s>' % (self.__class__.__name__, self.name)
...
Using auto
¶
Using auto
would look like:
>>> class Color(NoValue):
... RED = auto()
... BLUE = auto()
... GREEN = auto()
...
>>> Color.GREEN
<Color.GREEN>
Using object
¶
Using object
would look like:
>>> class Color(NoValue):
... RED = object()
... GREEN = object()
... BLUE = object()
...
>>> Color.GREEN
<Color.GREEN>
Using a descriptive string¶
Using a string as the value would look like:
>>> class Color(NoValue):
... RED = 'stop'
... GREEN = 'go'
... BLUE = 'too fast!'
...
>>> Color.GREEN
<Color.GREEN>
>>> Color.GREEN.value
'go'
Using a custom __new__()
¶
Using an auto-numbering __new__()
would look like:
>>> class AutoNumber(NoValue):
... 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
<Color.GREEN>
>>> Color.GREEN.value
2
To make a more general purpose AutoNumber
, add *args
to the signature:
>>> class AutoNumber(NoValue):
... def __new__(cls, *args): # this is the only change from above
... value = len(cls.__members__) + 1
... obj = object.__new__(cls)
... obj._value_ = value
... return obj
...
Then when you inherit from AutoNumber
you can write your own __init__
to handle any extra arguments:
>>> class Swatch(AutoNumber):
... def __init__(self, pantone='unknown'):
... self.pantone = pantone
... AUBURN = '3497'
... SEA_GREEN = '1246'
... BLEACHED_CORAL = () # New color, no Pantone code yet!
...
>>> Swatch.SEA_GREEN
<Swatch.SEA_GREEN: 2>
>>> Swatch.SEA_GREEN.pantone
'1246'
>>> Swatch.BLEACHED_CORAL.pantone
'unknown'
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
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'
참고
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.
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
TimePeriod¶
An example to show the _ignore_
attribute in use:
>>> from datetime import timedelta
>>> class Period(timedelta, Enum):
... "different lengths of time"
... _ignore_ = 'Period i'
... Period = vars()
... for i in range(367):
... Period['day_%d' % i] = i
...
>>> list(Period)[:2]
[<Period.day_0: datetime.timedelta(0)>, <Period.day_1: datetime.timedelta(days=1)>]
>>> list(Period)[-2:]
[<Period.day_365: datetime.timedelta(days=365)>, <Period.day_366: datetime.timedelta(days=366)>]
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_enum_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¶
Supported __dunder__
names¶
__members__
is a read-only ordered mapping of member_name
:member
items. It is only available on the class.
__new__()
, if specified, must create and return the enum members; it is
also a very good idea to set the member’s _value_
appropriately. Once
all the members are created it is no longer used.
Supported _sunder_
names¶
_name_
– name of the member_value_
– value of the member; can be set / modified in__new__
_missing_
– a lookup function used when a value is not found; may be overridden_ignore_
– a list of names, either as alist()
or astr()
, that will not be transformed into members, and will be removed from the final class_order_
– used in Python 2/3 code to ensure member order is consistent (class attribute, removed during class creation)_generate_next_value_
– used by the Functional API and byauto
to get an appropriate value for an enum member; may be overridden
버전 3.6에 추가: _missing_
, _order_
, _generate_next_value_
버전 3.7에 추가: _ignore_
To help keep Python 2 / Python 3 code in sync an _order_
attribute can
be provided. It will be checked against the actual order of the enumeration
and raise an error if the two do not match:
>>> class Color(Enum):
... _order_ = 'RED GREEN BLUE'
... RED = 1
... BLUE = 3
... GREEN = 2
...
Traceback (most recent call last):
...
TypeError: member order does not match _order_
참고
In Python 2 code the _order_
attribute is necessary as definition
order is lost before it can be recorded.
Enum
member type¶
Enum
members are instances of their Enum
class, and are
normally accessed as EnumClass.member
. Under certain circumstances they
can also be accessed as EnumClass.member.member
, but you should never do
this as that lookup may fail or, worse, return something besides the
Enum
member you are looking for (this is another good reason to use
all-uppercase names for members):
>>> class FieldTypes(Enum):
... name = 0
... value = 1
... size = 2
...
>>> FieldTypes.value.size
<FieldTypes.size: 2>
>>> FieldTypes.size.value
2
버전 3.5에서 변경.
Boolean value of Enum
classes and members¶
Enum
members that are mixed with non-Enum
types (such as
int
, str
, etc.) are evaluated according to the mixed-in
type’s rules; otherwise, all members evaluate as True
. To make your
own Enum’s boolean evaluation depend on the member’s value add the following to
your class:
def __bool__(self):
return bool(self.value)
Enum
classes with methods¶
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']
Combining members of Flag
¶
If a combination of Flag members is not named, the repr()
will include
all named flags and all named combinations of flags that are in the value:
>>> class Color(Flag):
... RED = auto()
... GREEN = auto()
... BLUE = auto()
... MAGENTA = RED | BLUE
... YELLOW = RED | GREEN
... CYAN = GREEN | BLUE
...
>>> Color(3) # named combination
<Color.YELLOW: 3>
>>> Color(7) # not named combination
<Color.CYAN|MAGENTA|BLUE|YELLOW|GREEN|RED: 7>