Enum HOWTO

Author

Ethan Furman <ethan at stoneleaf dot us>

Basic Enum Tutorial

An Enum is a set of symbolic names bound to unique values. They are similar to global variables, but they offer a more useful repr(), grouping, type-safety, and a few other features.

They are most useful when you have a variable that can take one of a limited selection of values. For example, the days of the week:

>>> from enum import Enum
>>> class Weekday(Enum):
...     MONDAY = 1
...     TUESDAY = 2
...     WEDNESDAY = 3
...     THURSDAY = 4
...     FRIDAY = 5
...     SATURDAY = 6
...     SUNDAY = 7

As you can see, creating an Enum is as simple as writing a class that inherits from Enum itself.

注釈

Case of Enum Members

Because Enums are used to represent constants we recommend using UPPER_CASE names for members, and will be using that style in our examples.

Depending on the nature of the enum a member's value may or may not be important, but either way that value can be used to get the corresponding member:

>>> Weekday(3)
Weekday.WEDNESDAY

As you can see, the repr() of a member shows the enum name and the member name. The str() on a member shows only its name:

>>> print(Weekday.THURSDAY)
THURSDAY

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

>>> type(Weekday.MONDAY)
<enum 'Weekday'>
>>> isinstance(Weekday.FRIDAY, Weekday)
True

Enum members have an attribute that contains just their name:

>>> print(Weekday.TUESDAY.name)
TUESDAY

Likewise, they have an attribute for their value:

>>> Weekday.WEDNESDAY.value
3

Unlike many languages that treat enumerations solely as name/value pairs, Python Enums can have behavior added. For example, datetime.date has two methods for returning the weekday: weekday() and isoweekday(). The difference is that one of them counts from 0-6 and the other from 1-7. Rather than keep track of that ourselves we can add a method to the Weekday enum to extract the day from the date instance and return the matching enum member:

@classmethod
def from_date(cls, date):
    return cls(date.isoweekday())

The complete Weekday enum now looks like this:

>>> class Weekday(Enum):
...     MONDAY = 1
...     TUESDAY = 2
...     WEDNESDAY = 3
...     THURSDAY = 4
...     FRIDAY = 5
...     SATURDAY = 6
...     SUNDAY = 7
...     #
...     @classmethod
...     def from_date(cls, date):
...         return cls(date.isoweekday())

Now we can find out what today is! Observe:

>>> from datetime import date
>>> Weekday.from_date(date.today())
Weekday.TUESDAY

Of course, if you're reading this on some other day, you'll see that day instead.

This Weekday enum is great if our variable only needs one day, but what if we need several? Maybe we're writing a function to plot chores during a week, and don't want to use a list -- we could use a different type of Enum:

>>> from enum import Flag
>>> class Weekday(Flag):
...     MONDAY = 1
...     TUESDAY = 2
...     WEDNESDAY = 4
...     THURSDAY = 8
...     FRIDAY = 16
...     SATURDAY = 32
...     SUNDAY = 64

We've changed two things: we're inherited from Flag, and the values are all powers of 2.

Just like the original Weekday enum above, we can have a single selection:

>>> first_week_day = Weekday.MONDAY
>>> first_week_day
Weekday.MONDAY

But Flag also allows us to combine several members into a single variable:

>>> weekend = Weekday.SATURDAY | Weekday.SUNDAY
>>> weekend
Weekday.SATURDAY|Weekday.SUNDAY

You can even iterate over a Flag variable:

>>> for day in weekend:
...     print(day)
SATURDAY
SUNDAY

Okay, let's get some chores set up:

>>> chores_for_ethan = {
...     'feed the cat': Weekday.MONDAY | Weekday.WEDNESDAY | Weekday.FRIDAY,
...     'do the dishes': Weekday.TUESDAY | Weekday.THURSDAY,
...     'answer SO questions': Weekday.SATURDAY,
...     }

And a function to display the chores for a given day:

>>> def show_chores(chores, day):
...     for chore, days in chores.items():
...         if day in days:
...             print(chore)
>>> show_chores(chores_for_ethan, Weekday.SATURDAY)
answer SO questions

In cases where the actual values of the members do not matter, you can save yourself some work and use auto() for the values:

>>> from enum import auto
>>> class Weekday(Flag):
...     MONDAY = auto()
...     TUESDAY = auto()
...     WEDNESDAY = auto()
...     THURSDAY = auto()
...     FRIDAY = auto()
...     SATURDAY = auto()
...     SUNDAY = auto()

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
>>> Color(3)
Color.BLUE

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

>>> Color['RED']
Color.RED
>>> Color['GREEN']
Color.GREEN

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: 'SQUARE' already defined as: 2

However, an enum member can have other names associated with it. Given two entries A and B with the same value (and A defined first), B is an alias for the member A. By-value lookup of the value of A will return the member A. By-name lookup of A will return the member A. By-name lookup of B will also return the member A:

>>> class Shape(Enum):
...     SQUARE = 2
...     DIAMOND = 1
...     CIRCLE = 3
...     ALIAS_FOR_SQUARE = 2
...
>>> Shape.SQUARE
Shape.SQUARE
>>> Shape.ALIAS_FOR_SQUARE
Shape.SQUARE
>>> Shape(2)
Shape.SQUARE

注釈

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, you can use the unique() decorator:

>>> 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()
...
>>> [member.value for member in Color]
[1, 2, 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()
...
>>> [member.value for member in Ordinal]
['NORTH', 'SOUTH', 'EAST', 'WEST']

注釈

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, Shape.DIAMOND, Shape.CIRCLE]

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)
('DIAMOND', Shape.DIAMOND)
('CIRCLE', Shape.CIRCLE)
('ALIAS_FOR_SQUARE', Shape.SQUARE)

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

Most of 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
>>> 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 any value(s) 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: MoreColor: cannot extend enumeration 'Color'

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.

注釈

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
>>> Animal.ANT.value
1
>>> list(Animal)
[Animal.ANT, Animal.BEE, Animal.CAT, Animal.DOG]

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 by default 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__)

警告

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.

バージョン 3.5 で変更: 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]

StrEnum

The second variation of Enum that is provided is also a subclass of str. Members of a StrEnum can be compared to strings; by extension, string enumerations of different types can also be compared to each other. StrEnum exists to help avoid the problem of getting an incorrect member:

>>> from enum import StrEnum
>>> class Directions(StrEnum):
...     NORTH = 'north',    # notice the trailing comma
...     SOUTH = 'south'

Before StrEnum, Directions.NORTH would have been the tuple ('north',).

バージョン 3.10 で追加.

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, if possible. However, as the name implies, IntFlag members also subclass int and can be used wherever an int is used.

注釈

Any operation on an IntFlag member besides the bit-wise operations will lose the IntFlag membership.

Bit-wise operations that result in invalid IntFlag values will lose the IntFlag membership. See FlagBoundary for details.

バージョン 3.6 で追加.

バージョン 3.10 で変更.

Sample IntFlag class:

>>> from enum import IntFlag
>>> class Perm(IntFlag):
...     R = 4
...     W = 2
...     X = 1
...
>>> Perm.R | Perm.W
Perm.R|Perm.W
>>> 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
>>> ~Perm.RWX
Perm(0)
>>> Perm(7)
Perm.RWX

注釈

Named combinations are considered aliases. Aliases do not show up during iteration, but can be returned from by-value lookups.

バージョン 3.10 で変更.

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)
>>> bool(Perm.R & Perm.X)
False

Because IntFlag members are also subclasses of int they can be combined with them (but may lose IntFlag membership:

>>> Perm.X | 4
Perm.R|Perm.X

>>> Perm.X | 8
9

注釈

The negation operator, ~, always returns an IntFlag member with a positive value:

>>> (~Perm.X).value == (Perm.R|Perm.W).value == 6
True

IntFlag members can also be iterated over:

>>> list(RW)
[Perm.R, Perm.W]

バージョン 3.10 で追加.

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.

バージョン 3.6 で追加.

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)
>>> 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

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
>>> bool(Color.BLACK)
False

Flag members can also be iterated over:

>>> purple = Color.RED | Color.BLUE
>>> list(purple)
[Color.RED, Color.BLUE]

バージョン 3.10 で追加.

注釈

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:

  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 type.

  3. 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.

  4. %-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.

  5. Formatted string literals, str.format(), and format() will use the mixed-in type's __format__() unless __str__() or __format__() is overridden in the subclass, in which case the overridden methods or Enum methods will be used. Use the !s and !r format codes to force usage of the Enum 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'])
PY

>>> print(Coordinate(3))
VY

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 a list or a str, 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 by auto to get an appropriate value for an enum member; may be overridden

注釈

For standard Enum classes the next value chosen is the last value seen incremented by one.

For Flag classes the next value chosen will be the next highest power-of-two, regardless of the last value seen.

バージョン 3.6 で追加: _missing_, _order_, _generate_next_value_

バージョン 3.7 で追加: _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_:
['RED', 'BLUE', 'GREEN']
['RED', 'GREEN', 'BLUE']

注釈

In Python 2 code the _order_ attribute is necessary as definition order is lost before it can be recorded.

_Private__names

Private names are not converted to enum members, but remain normal attributes.

バージョン 3.10 で変更.

Enum member type

Enum members are instances of their enum class, and are normally accessed as EnumClass.member. In Python versions 3.5 to 3.9 you could access members from other members -- this practice was discouraged, and in 3.12 Enum will return to not allowing it, while in 3.10 and 3.11 it will raise a DeprecationWarning:

>>> class FieldTypes(Enum):
...     name = 0
...     value = 1
...     size = 2
...
>>> FieldTypes.value.size       
DeprecationWarning: accessing one member from another is not supported,
  and will be disabled in 3.12
<FieldTypes.size: 2>

バージョン 3.5 で変更.

バージョン 3.10 で変更.

Creating members that are mixed with other data types

When subclassing other data types, such as int or str, with an Enum, all values after the = are passed to that data type's constructor. For example:

>>> class MyEnum(IntEnum):
...     example = '11', 16      # '11' will be interpreted as a hexadecimal
...                             # number
>>> MyEnum.example.value
17

Boolean value of Enum classes and members

Enum classes 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)

Plain Enum classes always evaluate as True.

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__', 'mass', 'name', 'radius', 'surface_gravity', 'value']

Combining members of Flag

Iterating over a combination of Flag members will only return the members that are comprised of a single bit:

>>> class Color(Flag):
...     RED = auto()
...     GREEN = auto()
...     BLUE = auto()
...     MAGENTA = RED | BLUE
...     YELLOW = RED | GREEN
...     CYAN = GREEN | BLUE
...
>>> Color(3)  # named combination
Color.YELLOW
>>> Color(7)      # not named combination
Color.RED|Color.GREEN|Color.BLUE

StrEnum and str.__str__()

An important difference between StrEnum and other Enums is the __str__() method; because StrEnum members are strings, some parts of Python will read the string data directly, while others will call str(). To make those two operations have the same result, StrEnum.__str__() will be the same as str.__str__() so that str(StrEnum.member) == StrEnum.member is true.

Flag and IntFlag minutia

Using the following snippet for our examples:

>>> class Color(IntFlag):
...     BLACK = 0
...     RED = 1
...     GREEN = 2
...     BLUE = 4
...     PURPLE = RED | BLUE
...     WHITE = RED | GREEN | BLUE
...

the following are true:

  • single-bit flags are canonical

  • multi-bit and zero-bit flags are aliases

  • only canonical flags are returned during iteration:

    >>> list(Color.WHITE)
    [Color.RED, Color.GREEN, Color.BLUE]
    
  • negating a flag or flag set returns a new flag/flag set with the corresponding positive integer value:

    >>> Color.BLUE
    Color.BLUE
    
    >>> ~Color.BLUE
    Color.RED|Color.GREEN
    
  • names of pseudo-flags are constructed from their members' names:

    >>> (Color.RED | Color.GREEN).name
    'RED|GREEN'
    
  • multi-bit flags, aka aliases, can be returned from operations:

    >>> Color.RED | Color.BLUE
    Color.PURPLE
    
    >>> Color(7)  # or Color(-1)
    Color.WHITE
    
    >>> Color(0)
    Color.BLACK
    
  • membership / containment checking has changed slightly -- zero valued flags are never considered to be contained:

    >>> Color.BLACK in Color.WHITE
    False
    

    otherwise, if all bits of one flag are in the other flag, True is returned:

    >>> Color.PURPLE in Color.WHITE
    True
    

There is a new boundary mechanism that controls how out-of-range / invalid bits are handled: STRICT, CONFORM, EJECT, and KEEP:

  • STRICT --> raises an exception when presented with invalid values

  • CONFORM --> discards any invalid bits

  • EJECT --> lose Flag status and become a normal int with the given value

  • KEEP --> keep the extra bits
    • keeps Flag status and extra bits

    • extra bits do not show up in iteration

    • extra bits do show up in repr() and str()

The default for Flag is STRICT, the default for IntFlag is EJECT, and the default for _convert_ is KEEP (see ssl.Options for an example of when KEEP is needed).

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 EnumType 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. EnumType 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. EnumType 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.

While Enum, IntEnum, StrEnum, Flag, and IntFlag 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 value

  • use instances of object as the value

  • use a descriptive string as the value

  • use a tuple as the value and a custom __new__() to replace the tuple with an int 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.

Using auto

Using auto would look like:

>>> class Color(Enum):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> Color.GREEN
<Color.GREEN>

Using object

Using object would look like:

>>> class Color(Enum):
...     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(Enum):
...     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(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
<Color.GREEN>
>>> Color.GREEN.value
2

To make a more general purpose AutoNumber, add *args to the signature:

>>> class AutoNumber(Enum):
...     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>
>>> Swatch.SEA_GREEN.pantone
'1246'
>>> Swatch.BLEACHED_CORAL.pantone
'unknown'

注釈

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.

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'

注釈

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, Period.day_1]
>>> list(Period)[-2:]
[Period.day_365, Period.day_366]

Conforming input to Flag

Creating a Flag enum that is more resilient out-of-bounds results to mathematical operations, you can use the FlagBoundary.CONFORM setting:

>>> from enum import Flag, CONFORM, auto
>>> class Weekday(Flag, boundary=CONFORM):
...     MONDAY = auto()
...     TUESDAY = auto()
...     WEDNESDAY = auto()
...     THURSDAY = auto()
...     FRIDAY = auto()
...     SATURDAY = auto()
...     SUNDAY = auto()
>>> today = Weekday.TUESDAY
>>> Weekday(today + 22)  # what day is three weeks from tomorrow?
>>> Weekday.WEDNESDAY

Subclassing EnumType

While most enum needs can be met by customizing Enum subclasses, either with class decorators or custom functions, EnumType can be subclassed to provide a different Enum experience.