枚举 class 实例的属性值的类型注释

Question

In python, I have the following instance of the Enum class:

class Colour(Enum):
    Red = "1"
    Blue = "2"
    Green = "3"

I have a function that takes a value of an attribute of this class, ie one of Colour.Red.value , Colour.Blue.value or Colour.Green.value :

def myfunc(input_str):
    return(input_str)

How do I define the type annotation for input_str in this case?

Pylance returns a warning when using input_str: Literal[Colour.Red.value, Colour.Blue.value, Colour.Green.value] : "Type arguments for "Literal" must be None, a literal value (int, bool, str, or bytes), or an enum value". I understand from PEP-0586 that Literal[Colour.Red, Colour.Blue, Colour.Green] would be allowed, however that does not seem to fit my use case.

Answer 1

There is no need to artificially restrict yourself and demand for functions to only accept enum values specifically.

If you want myfunc to do something with the value of the member of the Colour enum, you should still let it take the enum member as an argument:

from enum import Enum
from typing import Literal


class Colour(Enum):
    Red = "1"
    Blue = "2"
    Green = "3"


def f(colour: Colour) -> None:
    print(colour.value.center(5, "-"))


def g(colour: Literal[Colour.Blue, Colour.Green]) -> None:
    print(colour.value * 5)


f(Colour.Red)
g(Colour.Green)

Output:

--1--
33333

Moreover, if you want, you can mix in str specifically as the supertype for your enum members. Then they can essentially be treated as strings for most intents and purposes.

In this code I don't access the member .value :

from enum import Enum
from typing import Literal


class Colour(str, Enum):  # notice the order of inheritance
    Red = "1"
    Blue = "2"
    Green = "3"


def f(colour: Colour) -> None:
    print(colour.center(5, "-"))  # `colour` is of type `str`


def g(colour: Literal[Colour.Blue, Colour.Green]) -> None:
    print(colour * 5)  # `colour` is of type `str`


f(Colour.Red)
g(Colour.Green)

Same output.

The order of inheritance actually guarantees that all str methods will be present and work as expected on all enum members.

It would surprise me, if you could not use those enum members anywhere in place of "regular" str objects.

You can verify the type, if you do isinstance(Colour.Red, str) . The members are now subtypes of str . If you want some other concrete data type for the members, you can use that too.