10.4. Enum Auto

  • Automatically generated value

  • auto()

auto can be used in place of a value. If used, the Enum machinery will call an Enum's _generate_next_value_() to get an appropriate value. For Enum and IntEnum that appropriate value will be the last value plus one; for Flag and IntFlag it will be the first power-of-two greater than the last value; for StrEnum it will be the lower-cased version of the member's name. Care must be taken if mixing auto() with manually specified values.

auto instances are only resolved when at the top level of an assignment:

  • FIRST = auto() will work (auto() is replaced with 1);

  • SECOND = auto(), -2 will work (auto is replaced with 2, so 2, -2 is used to create the SECOND enum member;

  • THREE = [auto(), -3] will not work (<auto instance>, -3 is used to create the THREE enum member)

_generate_next_value_ can be overridden to customize the values used by auto.

Note

in 3.13 the default "generate_next_value_ will always return the highest member value incremented by 1, and will fail if any member is an incompatible type.

>>> from enum import IntEnum, auto
>>>
>>> class Status(IntEnum):
...     TODO = auto()
...     IN_PROGRESS = auto()
...     IN_REVIEW = auto()
...     DONE = auto()
...     REJECTED = auto()
>>>
>>>
>>> Status.DONE.value
4

Let's add "In Test" status in the middle of an Enum:

>>> class Status(IntEnum):
...     TODO = auto()
...     IN_PROGRESS = auto()
...     IN_TEST = auto()
...     IN_REVIEW = auto()
...     DONE = auto()
...     REJECTED = auto()
>>>
>>> Status.DONE.value
5

Let's change Status to StrEnum

>>> from enum import StrEnum, auto
>>>
>>> class Status(StrEnum):
...     TODO = auto()
...     IN_PROGRESS = auto()
...     IN_TEST = auto()
...     IN_REVIEW = auto()
...     DONE = auto()
...     REJECTED = auto()
>>>
>>> Status.DONE.value
'done'
>>>
>>> Status.IN_PROGRESS.value
'in_progress'

10.4.1. SetUp

>>> from enum import StrEnum, IntEnum, auto

10.4.2. StrEnum

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

>>> Color.RED
<Color.RED: 'red'>

10.4.3. IntEnum

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

>>> Color.RED
<Color.RED: 1>

10.4.4. Use Case - 1

>>> class Animal(StrEnum):
...     ANT = auto()
...     BEE = auto()
...     CAT = auto()
...     DOG = auto()