How to override iteration behavior of a enum Flag subclass?

Question

I'm trying to override iteration behavior for a Flag enum:

from enum import Flag, auto

class MyFlags(Flag):
    A = auto()
    B = auto()
    C = auto()
    
    D = A | C

    def __iter__(self):
       for x in super().__iter__():
           if x.value % 2 == 0:
                yield x

But it seems the __iter__ is never actually called:

>>> list(MyFlags)
[MyFlags.A, MyFlags.B, MyFlags.C, MyFlags.D]

How can I override iteration behavior here?

Answer 1

You can sublass EnumMeta and then add metaclass= to your Enum class:

from enum import Flag, EnumMeta, auto


class MyEnumMeta(EnumMeta):
    def __iter__(self):
       for x in super().__iter__():
           if x.value % 2 == 0:
                yield x


class MyFlags(Flag, metaclass=MyEnumMeta):
    A = auto()
    B = auto()
    C = auto()
    
    D = A | C


print(list(MyFlags))

Prints:

[<MyFlags.B: 2>, <MyFlags.C: 4>]