Enum HOWTO (original) (raw)

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

Or perhaps the RGB primary colors:

from enum import Enum class Color(Enum): ... RED = 1 ... GREEN = 2 ... BLUE = 3

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

Note

Case of Enum Members

Because Enums are used to represent constants, and to help avoid issues with name clashes between mixin-class methods/attributes and enum names, we strongly 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: 3>

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

print(Weekday.THURSDAY) Weekday.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.datehas 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 Weekdayenum 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: 2>

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: 1>

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

weekend = Weekday.SATURDAY | Weekday.SUNDAY weekend <Weekday.SATURDAY|SUNDAY: 96>

You can even iterate over a Flag variable:

for day in weekend: ... print(day) Weekday.SATURDAY Weekday.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() ... WEEKEND = SATURDAY | SUNDAY

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: '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), Bis 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: 2> Shape.ALIAS_FOR_SQUARE <Shape.SQUARE: 2> Shape(2) <Shape.SQUARE: 2>

Note

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): ... @staticmethod ... 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']

Iteration

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

list(Shape) [<Shape.SQUARE: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>] list(Weekday) [<Weekday.MONDAY: 1>, <Weekday.TUESDAY: 2>, <Weekday.WEDNESDAY: 4>, <Weekday.THURSDAY: 8>, <Weekday.FRIDAY: 16>, <Weekday.SATURDAY: 32>, <Weekday.SUNDAY: 64>]

Note that the aliases Shape.ALIAS_FOR_SQUARE and Weekday.WEEKEND aren’t shown.

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']

Note

Aliases for flags include values with multiple flags set, such as 3, and no flags set, i.e. 0.

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 "", line 1, in 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

Warning

It is possible to reload modules – if a reloaded module contains enums, they will be recreated, and the new members may not compare identical/equal to the original members.

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: 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__(), any value(s) given to the enum member will be passed into those methods. See Planet for an example.

Note

The __new__() method, if defined, is used during creation of the Enum members; it is then replaced by Enum’s __new__() which is used after class creation for lookup of existing members. See When to use __new__() vs. __init__() for more details.

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: <enum 'MoreColor'> cannot extend <enum '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.)

Dataclass support

When inheriting from a dataclass, the __repr__() omits the inherited class’ name. For example:

from dataclasses import dataclass, field @dataclass ... class CreatureDataMixin: ... size: str ... legs: int ... tail: bool = field(repr=False, default=True) ... class Creature(CreatureDataMixin, Enum): ... BEETLE = 'small', 6 ... DOG = 'medium', 4 ... Creature.DOG <Creature.DOG: size='medium', legs=4>

Use the dataclass() argument repr=Falseto use the standard repr().

Changed in version 3.12: Only the dataclass fields are shown in the value area, not the dataclass’ name.

Note

Adding dataclass() decorator to Enumand its subclasses is not supported. It will not raise any errors, but it will produce very strange results at runtime, such as members being equal to each other:

@dataclass # don't do this: it does not make any sense ... class Color(Enum): ... RED = 1 ... BLUE = 2 ... Color.RED is Color.BLUE False Color.RED == Color.BLUE # problem is here: they should not be equal True

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.

Note

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. The default method is by-value, but enums with complicated values may want to use by-name:

import enum class MyEnum(enum.Enum): ... reduce_ex = enum.pickle_by_enum_name

Note

Using by-name for flags is not recommended, as unnamed aliases will not unpickle.

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> 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 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 a 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)

Warning

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=, start=1, )

Changed in version 3.5: The start parameter was added.

Derived Enumerations

IntEnum

The first variation of Enum that is provided is also a subclass ofint. 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 ofstr. Members of a StrEnum can be compared to strings; by extension, string enumerations of different types can also be compared to each other.

Added in version 3.11.

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 anIntFlag member, if possible. Like IntEnum, IntFlagmembers are also integers and can be used wherever an int is used.

Note

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 theIntFlag membership. See FlagBoundary for details.

Added in version 3.6.

Changed in version 3.11.

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: 0> Perm(7) <Perm.RWX: 7>

Note

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

Changed in version 3.11.

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|X: 5>

Perm.X + 8 9

Note

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: 4>, <Perm.W: 2>]

Added in version 3.11.

Flag

The last variation is Flag. Like IntFlag, Flagmembers can be combined using the bitwise operators (&, |, ^, ~). UnlikeIntFlag, 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.

Added in version 3.6.

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 will not:

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

Flag members can also be iterated over:

purple = Color.RED | Color.BLUE list(purple) [<Color.RED: 1>, <Color.BLUE: 2>]

Added in version 3.11.

Note

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). IntEnumand IntFlag should be used only in cases where Enum andFlag 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, ReprEnum): # or Enum instead of ReprEnum pass

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

Some rules:

  1. When subclassing Enum, mix-in types must appear before theEnum class itself in the sequence of bases, as in the IntEnumexample above.
  2. Mix-in types must be subclassable. For example, bool andrange are not subclassable and will throw an error during Enum creation if used as the mix-in type.
  3. 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.
  4. 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.
  5. A data type is a mixin that defines __new__(), or adataclass
  6. %-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.
  7. Formatted string literals, str.format(), and format() will use the enum’s __str__() method.

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

Warning

Do not call super().__new__(), as the lookup-only __new__ is the one that is found; instead, use the data type directly.

Finer Points

Supported __dunder__ names

__members__ is a read-only ordered mapping of member_name:memberitems. 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

Added in version 3.6: _missing_, _order_, _generate_next_value_

Added in version 3.7: _ignore_

Added in version 3.13: _add_alias_, _add_value_alias_

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']

Note

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.

Changed in version 3.11.

Enum member type

Enum members are instances of their enum class, and are normally accessed asEnumClass.member. In certain situations, such as writing custom enum behavior, being able to access one member directly from another is useful, and is supported; however, in order to avoid name clashes between member names and attributes/methods from mixed-in classes, upper-case names are strongly recommended.

Changed in version 3.5.

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): # help(int) -> int(x, base=10) -> integer ... example = '11', 16 # so x='11' and base=16 ... MyEnum.example.value # and hex(11) is... 17

Boolean value of Enum classes and members

Enum classes that are mixed with non-Enum types (such asint, 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 Planetclass below, 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: 3> Color(7) # not named combination <Color.RED|GREEN|BLUE: 7>

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:

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

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 and Flags different?

Enums have a custom metaclass that affects many aspects of both derived Enumclasses 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 Enumclass are correct (such as __new__(), __getnewargs__(),__str__() and __repr__()).

Flag Classes

Flags have an expanded view of aliasing: to be canonical, the value of a flag needs to be a power-of-two value, and not a duplicate name. So, in addition to theEnum definition of alias, a flag with no value (a.k.a. 0) or with more than one power-of-two value (e.g. 3) is considered an alias.

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.

Flag Members

Flag members can be iterated over just like the Flag class, and only the canonical members will be returned. For example:

list(Color) [<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]

(Note that BLACK, PURPLE, and WHITE do not show up.)

Inverting a flag member returns the corresponding positive value, rather than a negative value — for example:

~Color.RED <Color.GREEN|BLUE: 6>

Flag members have a length corresponding to the number of power-of-two values they contain. For example:

Enum Cookbook

While Enum, IntEnum, StrEnum, Flag, andIntFlag 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:

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: 3>

Using object

Using object would look like:

class Color(Enum): ... RED = object() ... GREEN = object() ... BLUE = object() ... Color.GREEN <Color.GREEN: <object object at 0x...>>

This is also a good example of why you might want to write your own__repr__():

class Color(Enum): ... RED = object() ... GREEN = object() ... BLUE = object() ... def repr(self): ... return "<%s.%s>" % (self.class.name, self.name) ... 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: '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: 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: 2> Swatch.SEA_GREEN.pantone '1246' Swatch.BLEACHED_CORAL.pantone 'unknown'

Note

The __new__() method, if defined, is used during creation of the Enum members; it is then replaced by Enum’s __new__() which is used after class creation for lookup of existing members.

Warning

Do not call super().__new__(), as the lookup-only __new__ is the one that is found; instead, use the data type directly – e.g.:

obj = int.new(cls, value)

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 value 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'

Note

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.

MultiValueEnum

Supports having more than one value per member:

class MultiValueEnum(Enum): ... def new(cls, value, *values): ... self = object.new(cls) ... self.value = value ... for v in values: ... self.add_value_alias(v) ... return self ... class DType(MultiValueEnum): ... float32 = 'f', 8 ... double64 = 'd', 9 ... DType('f') <DType.float32: 'f'> DType(9) <DType.double64: 'd'>

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)>]

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.