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Q&A

Is it possible to write protocols for enumerations in Python?

+4
−0

Having recently learned about protocols in Python, I now wonder if you can write a protocol for enumerations. That is, a protocol that says that you are supposed to pass an enum that has certain items with certain values, but doesn't specify a specific enum.

To explain how I mean it, here's some non-working (in Python 3.8) code that demonstrates how I imagine it to work:

from typing import Protocol
from enum import Enum

# The following does NOT work; the question is whether there's
# something that does work.
class EnumProtocol(Enum, Protocol):
    FOO = 1
    BAR = 2

# The above gives TypeError: metaclass conflict

# this is how it is supposed to work

# A function taking an argument with this protocol
def useit(foobar: EnumProtocol) -> None:
    if foobar = EnumProtocol.FOO:
        print("foo")
    elif foobar = EnumProtocol.BAR:
        print("bar")
    else:
        print(f"{foobar.name} = {foobar.value}")

# The following class fulfils the protocol
class GoodEnum(Enum):
    FOO = 1
    BAR = 2
    BAZ = 3

useit(GoodEnum.FOO) # ok, prints "foo"
useit(GoodEnum.BAZ) # ok, prints "BAZ = 3"

# The following class violates the protocol by having wrong values
class BadEnum(Enum):
    FOO = 2
    BAR = 3

useit(BadEnum.FOO) # type checking gives error

# The following class violates the protocol by missing BAR
class UglyEnum(Enum):
    FOO = 1
    BAZ = 3

useit(UglyEnum.FOO) # type checking gives error
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1 answer

+5
−0

There's one big difficulty with the proposed pattern. Enum instances are singleton instances of their particular class, and in general two enums from different classes are not equal even if they wrap the same int value. So an expression like foobar == EnumProtocol.FOO is never going to be true if foobar is any enum instance.

If you're okay with adapting that to foobar.value == EnumProtocol.FOO, then you can do this:

from typing import Literal, Protocol

from enum import Enum

class EnumProtocol(Protocol):
    FOO: Literal[1] = 1
    BAR: Literal[2] = 2

    # You could move these properties into a superprotocol
    @property
    def name(self) -> str:
        pass
    
    @property
    def value(self) -> int:
        pass

def useit(foobar: EnumProtocol) -> None:
    if foobar.value == EnumProtocol.FOO:
        print("foo")
    elif foobar.value == EnumProtocol.BAR:
        print("bar")
    else:
        print(f"{foobar.name} = {foobar.value}")

# The following class fulfils the protocol
class GoodEnum(Enum):
    FOO = 1
    BAR = 2
    BAZ = 3

useit(GoodEnum.FOO) # ok, prints "foo"
useit(GoodEnum.BAZ) # ok, prints "BAZ = 3"

# The following class violates the protocol by having wrong values
class BadEnum(Enum):
    FOO = 2
    BAR = 3

useit(BadEnum.FOO) # type checking gives error

# The following class violates the protocol by missing BAR
class UglyEnum(Enum):
    FOO = 1
    BAZ = 3

useit(UglyEnum.FOO) # type checking gives error
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You might want to add some details to your flag.

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