Spaces:

ITookAPill
/

PyComp

Running on Zero

App Files Files Community

PyComp / python_doc_md /Beginner /howto /enum.md

ITookAPill

PyComp First Commit

9273228 5 months ago

preview code

Raw

History Blame Contribute Delete

48.7 kB

A newer version of the Gradio SDK is available: 6.20.0

Upgrade

Enum HOWTO

:::: {#enum-basic-tutorial} ::: currentmodule enum ::: ::::

An Enum{.interpreted-text role="class"} is a set of symbolic names bound to unique values. They are similar to global variables, but they offer a more useful repr{.interpreted-text role="func"}, 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{.interpreted-text role="class"} is as simple as writing a class that inherits from Enum{.interpreted-text role="class"} itself.

:::: note ::: title 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{.interpreted-text role="attr"}:

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

Likewise, they have an attribute for their !value{.interpreted-text role="attr"}:

>>> 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{.interpreted-text role="class"} has two methods for returning the weekday: ~datetime.date.weekday{.interpreted-text role="meth"} and ~datetime.date.isoweekday{.interpreted-text role="meth"}. 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{.interpreted-text role="class"} enum to extract the day from the ~datetime.date{.interpreted-text role="class"} instance and return the matching enum member:

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

The complete !Weekday{.interpreted-text role="class"} 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())     # doctest: +SKIP
<Weekday.TUESDAY: 2>

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

This !Weekday{.interpreted-text role="class"} 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{.interpreted-text role="class"} -- we could use a different type of Enum{.interpreted-text role="class"}:

>>> 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{.interpreted-text role="class"}, and the values are all powers of 2.

Just like the original !Weekday{.interpreted-text role="class"} enum above, we can have a single selection:

>>> first_week_day = Weekday.MONDAY
>>> first_week_day
<Weekday.MONDAY: 1>

But Flag{.interpreted-text role="class"} 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{.interpreted-text role="class"} 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{.interpreted-text role="func"} 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 {#enum-advanced-tutorial}

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{.interpreted-text role="attr"} or !value{.interpreted-text role="attr"}:

>>> 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: 2>
>>> Shape.ALIAS_FOR_SQUARE
<Shape.SQUARE: 2>
>>> Shape(2)
<Shape.SQUARE: 2>

:::: note ::: title 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{.interpreted-text role="func"} 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{.interpreted-text role="class"}:

>>> 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 ~Enum._generate_next_value_{.interpreted-text role="func"}, 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']

:::: note ::: title Note :::

The ~Enum._generate_next_value_{.interpreted-text role="meth"} 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>]
>>> 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 ::: title 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 "<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{.interpreted-text role="class"} was explicitly designed to behave differently, see below):

>>> Color.BLUE == 2
False

:::: warning ::: title 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 (~object.__str__{.interpreted-text role="meth"}, ~object.__add__{.interpreted-text role="meth"}, etc.), descriptors (methods are also descriptors), and variable names listed in ~Enum._ignore_{.interpreted-text role="attr"}.

Note: if your enumeration defines ~object.__new__{.interpreted-text role="meth"} and/or ~object.__init__{.interpreted-text role="meth"}, any value(s) given to the enum member will be passed into those methods. See Planet for an example.

:::: note ::: title Note :::

The ~object.__new__{.interpreted-text role="meth"} method, if defined, is used during creation of the Enum members; it is then replaced by Enum's ~object.__new__{.interpreted-text role="meth"} which is used after class creation for lookup of existing members. See new-vs-init{.interpreted-text role="ref"} for more details. ::::

Restricted Enum subclassing

A new Enum{.interpreted-text role="class"} class must have one base enum class, up to one concrete data type, and as many object{.interpreted-text role="class"}-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 {#enum-dataclass-support}

When inheriting from a ~dataclasses.dataclass{.interpreted-text role="class"}, the ~Enum.__repr__{.interpreted-text role="meth"} 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 ~dataclasses.dataclass{.interpreted-text role="func"} argument repr=False to use the standard repr{.interpreted-text role="func"}.

::: versionchanged 3.12 Only the dataclass fields are shown in the value area, not the dataclass' name. :::

:::: note ::: title Note :::

Adding ~dataclasses.dataclass{.interpreted-text role="func"} decorator to Enum{.interpreted-text role="class"} and 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 ::: title 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 ~object.__reduce_ex__{.interpreted-text role="meth"} 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 ::: title Note :::

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

Functional API

The Enum{.interpreted-text role="class"} 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 ~collections.namedtuple{.interpreted-text role="class"}. The first argument of the call to Enum{.interpreted-text role="class"} 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{.interpreted-text role="class"} is returned. In other words, the above assignment to !Animal{.interpreted-text role="class"} 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 ::: title 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 ~type.__qualname__{.interpreted-text role="attr"} 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.

::: versionchanged 3.5 The start parameter was added. :::

Derived Enumerations

IntEnum

The first variation of Enum{.interpreted-text role="class"} that is provided is also a subclass of int{.interpreted-text role="class"}. Members of an IntEnum{.interpreted-text role="class"} 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{.interpreted-text role="class"} enumerations:

>>> class Shape(IntEnum):
...     CIRCLE = 1
...     SQUARE = 2
...
>>> class Color(Enum):
...     RED = 1
...     GREEN = 2
...
>>> Shape.CIRCLE == Color.RED
False

IntEnum{.interpreted-text role="class"} 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{.interpreted-text role="class"} that is provided is also a subclass of str{.interpreted-text role="class"}. Members of a StrEnum{.interpreted-text role="class"} can be compared to strings; by extension, string enumerations of different types can also be compared to each other.

::: versionadded 3.11 :::

IntFlag

The next variation of Enum{.interpreted-text role="class"} provided, IntFlag{.interpreted-text role="class"}, is also based on int{.interpreted-text role="class"}. The difference being IntFlag{.interpreted-text role="class"} members can be combined using the bitwise operators (&, |, ^, ~) and the result is still an IntFlag{.interpreted-text role="class"} member, if possible. Like IntEnum{.interpreted-text role="class"}, IntFlag{.interpreted-text role="class"} members are also integers and can be used wherever an int{.interpreted-text role="class"} is used.

:::: note ::: title Note :::

Any operation on an IntFlag{.interpreted-text role="class"} member besides the bit-wise operations will lose the IntFlag{.interpreted-text role="class"} membership.

Bit-wise operations that result in invalid IntFlag{.interpreted-text role="class"} values will lose the IntFlag{.interpreted-text role="class"} membership. See FlagBoundary{.interpreted-text role="class"} for details. ::::

::: versionadded 3.6 :::

::: versionchanged 3.11 :::

Sample IntFlag{.interpreted-text role="class"} 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 ::: title Note :::

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

::: versionchanged 3.11 :::

Another important difference between IntFlag{.interpreted-text role="class"} and Enum{.interpreted-text role="class"} is that if no flags are set (the value is 0), its boolean evaluation is False{.interpreted-text role="data"}:

>>> Perm.R & Perm.X
<Perm: 0>
>>> bool(Perm.R & Perm.X)
False

Because IntFlag{.interpreted-text role="class"} members are also subclasses of int{.interpreted-text role="class"} they can be combined with them (but may lose IntFlag{.interpreted-text role="class"} membership:

>>> Perm.X | 4
<Perm.R|X: 5>

>>> Perm.X + 8
9

:::: note ::: title Note :::

The negation operator, ~, always returns an IntFlag{.interpreted-text role="class"} member with a positive value:

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

::::

IntFlag{.interpreted-text role="class"} members can also be iterated over:

>>> list(RW)
[<Perm.R: 4>, <Perm.W: 2>]

::: versionadded 3.11 :::

Flag

The last variation is Flag{.interpreted-text role="class"}. Like IntFlag{.interpreted-text role="class"}, Flag{.interpreted-text role="class"} members can be combined using the bitwise operators (&, |, ^, ~). Unlike IntFlag{.interpreted-text role="class"}, they cannot be combined with, nor compared against, any other Flag{.interpreted-text role="class"} enumeration, nor int{.interpreted-text role="class"}. While it is possible to specify the values directly it is recommended to use auto{.interpreted-text role="class"} as the value and let Flag{.interpreted-text role="class"} select an appropriate value.

::: versionadded 3.6 :::

Like IntFlag{.interpreted-text role="class"}, if a combination of Flag{.interpreted-text role="class"} members results in no flags being set, the boolean evaluation is False{.interpreted-text role="data"}:

>>> 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{.interpreted-text role="class"} members can also be iterated over:

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

::: versionadded 3.11 :::

:::: note ::: title Note :::

For the majority of new code, Enum{.interpreted-text role="class"} and Flag{.interpreted-text role="class"} are strongly recommended, since IntEnum{.interpreted-text role="class"} and IntFlag{.interpreted-text role="class"} break some semantic promises of an enumeration (by being comparable to integers, and thus by transitivity to other unrelated enumerations). IntEnum{.interpreted-text role="class"} and IntFlag{.interpreted-text role="class"} should be used only in cases where Enum{.interpreted-text role="class"} and Flag{.interpreted-text role="class"} will not do; for example, when integer constants are replaced with enumerations, or for interoperability with other systems. ::::

Others

While IntEnum{.interpreted-text role="class"} is part of the enum{.interpreted-text role="mod"} 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{.interpreted-text role="class"} that mixes in float{.interpreted-text role="class"} instead of int{.interpreted-text role="class"}.

Some rules:

When subclassing Enum{.interpreted-text role="class"}, mix-in types must appear before the Enum{.interpreted-text role="class"} class itself in the sequence of bases, as in the IntEnum{.interpreted-text role="class"} example above.
Mix-in types must be subclassable. For example, bool{.interpreted-text role="class"} and range{.interpreted-text role="class"} are not subclassable and will throw an error during Enum creation if used as the mix-in type.
While Enum{.interpreted-text role="class"} 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{.interpreted-text role="class"} above. This restriction does not apply to mix-ins which only add methods and don't specify another type.
When another data type is mixed in, the ~Enum.value{.interpreted-text role="attr"} attribute is not the same as the enum member itself, although it is equivalent and will compare equal.
A data type is a mixin that defines ~object.__new__{.interpreted-text role="meth"}, or a ~dataclasses.dataclass{.interpreted-text role="class"}
%-style formatting: %s and %r call the Enum{.interpreted-text role="class"} class's ~object.__str__{.interpreted-text role="meth"} and ~object.__repr__{.interpreted-text role="meth"} respectively; other codes (such as %i or %h for IntEnum) treat the enum member as its mixed-in type.
Formatted string literals <f-strings>{.interpreted-text role="ref"}, str.format{.interpreted-text role="meth"}, and format{.interpreted-text role="func"} will use the enum's ~object.__str__{.interpreted-text role="meth"} method.

:::: note ::: title Note :::

Because IntEnum{.interpreted-text role="class"}, IntFlag{.interpreted-text role="class"}, and StrEnum{.interpreted-text role="class"} are designed to be drop-in replacements for existing constants, their ~object.__str__{.interpreted-text role="meth"} method has been reset to their data types' ~object.__str__{.interpreted-text role="meth"} method. ::::

When to use `~object.new`{.interpreted-text role="meth"} vs. `~object.init`{.interpreted-text role="meth"} {#new-vs-init}

~object.__new__{.interpreted-text role="meth"} must be used whenever you want to customize the actual value of the Enum{.interpreted-text role="class"} member. Any other modifications may go in either ~object.__new__{.interpreted-text role="meth"} or ~object.__init__{.interpreted-text role="meth"}, with ~object.__init__{.interpreted-text role="meth"} 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 ::: title 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` and `_sunder_` names

The supported __dunder__ and _sunder_ names can be found in the Enum API documentation <enum-dunder-sunder>{.interpreted-text role="ref"}.

[Private]{#private}__names

Private names <private-name-mangling>{.interpreted-text role="ref"} are not converted to enum members, but remain normal attributes.

::: versionchanged 3.11 :::

`Enum` member type

Enum members are instances of their enum class, and are normally accessed as EnumClass.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.

::: versionchanged 3.5 :::

Creating members that are mixed with other data types

When subclassing other data types, such as int{.interpreted-text role="class"} or str{.interpreted-text role="class"}, with an Enum{.interpreted-text role="class"}, 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{.interpreted-text role="class"} types (such as int{.interpreted-text role="class"}, str{.interpreted-text role="class"}, etc.) are evaluated according to the mixed-in type's rules; otherwise, all members evaluate as True{.interpreted-text role="data"}. 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{.interpreted-text role="class"} classes always evaluate as True{.interpreted-text role="data"}.

`Enum` classes with methods

If you give your enum subclass extra methods, like the Planet class below, those methods will show up in a dir{.interpreted-text role="func"} of the member, but not of the class:

>>> dir(Planet)                         # doctest: +SKIP
['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__']
>>> dir(Planet.EARTH)                   # doctest: +SKIP
['__class__', '__doc__', '__module__', 'mass', 'name', 'radius', 'surface_gravity', 'value']

Combining members of `Flag`

Iterating over a combination of Flag{.interpreted-text role="class"} 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:

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: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]

negating a flag or flag set returns a new flag/flag set with the corresponding positive integer value:
```
>>> Color.BLUE
<Color.BLUE: 4>

>>> ~Color.BLUE
<Color.RED|GREEN: 3>
```

names of pseudo-flags are constructed from their members' names:

>>> (Color.RED | Color.GREEN).name
'RED|GREEN'

>>> class Perm(IntFlag):
...     R = 4
...     W = 2
...     X = 1
...
>>> (Perm.R & Perm.W).name is None  # effectively Perm(0)
True

multi-bit flags, aka aliases, can be returned from operations:

>>> Color.RED | Color.BLUE
<Color.PURPLE: 5>

>>> Color(7)  # or Color(-1)
<Color.WHITE: 7>

>>> Color(0)
<Color.BLACK: 0>

membership / containment checking: zero-valued flags are always considered to be contained:
```
>>> Color.BLACK in Color.WHITE
True
```
otherwise, only if all bits of one flag are in the other flag will True be returned:
```
>>> Color.PURPLE in Color.WHITE
True

>>> Color.GREEN in Color.PURPLE
False
```

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 and Flags different? {#enum-class-differences}

Enums have a custom metaclass that affects many aspects of both derived Enum{.interpreted-text role="class"} classes and their instances (members).

Enum Classes

The EnumType{.interpreted-text role="class"} metaclass is responsible for providing the ~object.__contains__{.interpreted-text role="meth"}, ~object.__dir__{.interpreted-text role="meth"}, ~object.__iter__{.interpreted-text role="meth"} and other methods that allow one to do things with an Enum{.interpreted-text role="class"} class that fail on a typical class, such as list(Color) or some_enum_var in Color. EnumType{.interpreted-text role="class"} is responsible for ensuring that various other methods on the final Enum{.interpreted-text role="class"} class are correct (such as ~object.__new__{.interpreted-text role="meth"}, ~object.__getnewargs__{.interpreted-text role="meth"}, ~object.__str__{.interpreted-text role="meth"} and ~object.__repr__{.interpreted-text role="meth"}).

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 the Enum{.interpreted-text role="class"} 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{.interpreted-text role="class"} creates them all while it is creating the enum class itself, and then puts a custom ~object.__new__{.interpreted-text role="meth"} 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{.interpreted-text role="class"} 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:

>>> len(Color.PURPLE)
2

Enum Cookbook

While Enum{.interpreted-text role="class"}, IntEnum{.interpreted-text role="class"}, StrEnum{.interpreted-text role="class"}, Flag{.interpreted-text role="class"}, and IntFlag{.interpreted-text role="class"} 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{.interpreted-text role="class"} for the value
use instances of object{.interpreted-text role="class"} as the value
use a descriptive string as the value
use a tuple as the value and a custom ~object.__new__{.interpreted-text role="meth"} to replace the tuple with an int{.interpreted-text role="class"} 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`{.interpreted-text role="class"}

Using auto{.interpreted-text role="class"} would look like:

>>> class Color(Enum):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> Color.GREEN
<Color.GREEN: 3>

Using `object`{.interpreted-text role="class"}

Using object{.interpreted-text role="class"} would look like:

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

This is also a good example of why you might want to write your own ~object.__repr__{.interpreted-text role="meth"}:

>>> 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 `~object.new`{.interpreted-text role="meth"}

Using an auto-numbering ~object.__new__{.interpreted-text role="meth"} 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 ::: title Note :::

:::: warning ::: title 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{.interpreted-text role="class"} and so maintains the normal Enum{.interpreted-text role="class"} 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 ::: title 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{.interpreted-text role="func"} decorator can be used instead. ::::

MultiValueEnum {#multi-value-enum}

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 ~object.__new__{.interpreted-text role="meth"} or ~object.__init__{.interpreted-text role="meth"} 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 {#enum-time-period}

An example to show the ~Enum._ignore_{.interpreted-text role="attr"} 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 {#enumtype-examples}

While most enum needs can be met by customizing Enum{.interpreted-text role="class"} subclasses, either with class decorators or custom functions, EnumType{.interpreted-text role="class"} can be subclassed to provide a different Enum experience.

Enum HOWTO

Programmatic access to enumeration members and their attributes {#enum-advanced-tutorial}

Duplicating enum members and values

Ensuring unique enumeration values

Using automatic values

Iteration

Comparisons

Allowed members and attributes of enumerations

Restricted Enum subclassing

Dataclass support {#enum-dataclass-support}

Pickling

Functional API

Derived Enumerations

IntEnum

StrEnum

IntFlag

Flag

Others

When to use ~object.__new__{.interpreted-text role="meth"} vs. ~object.__init__{.interpreted-text role="meth"} {#new-vs-init}

Finer Points

Supported __dunder__ and _sunder_ names

[Private]{#private}__names

Enum member type

Creating members that are mixed with other data types

Boolean value of Enum classes and members

Enum classes with methods

Combining members of Flag

Flag and IntFlag minutia

How are Enums and Flags different? {#enum-class-differences}

Enum Classes

Flag Classes

Enum Members (aka instances)

Flag Members

Enum Cookbook

Omitting values

Using auto{.interpreted-text role="class"}

Using object{.interpreted-text role="class"}

Using a descriptive string

Using a custom ~object.__new__{.interpreted-text role="meth"}

OrderedEnum

DuplicateFreeEnum

MultiValueEnum {#multi-value-enum}

Planet

TimePeriod {#enum-time-period}

Subclassing EnumType {#enumtype-examples}

When to use `~object.new`{.interpreted-text role="meth"} vs. `~object.init`{.interpreted-text role="meth"} {#new-vs-init}

Supported `dunder` and `_sunder_` names

`Enum` member type

Boolean value of `Enum` classes and members

`Enum` classes with methods

Combining members of `Flag`

`Flag` and `IntFlag` minutia

Using `auto`{.interpreted-text role="class"}

Using `object`{.interpreted-text role="class"}

Using a custom `~object.new`{.interpreted-text role="meth"}