Allow enum with zero members (rational provided)

[Hibou57]

Feature

Just an option to bypass an error message and allow this:

class E(enum.IntEnum):
    pass

E1: E
E2: E
E3: E

Pitch

Compared to this, it has the advantage below:

class E(enum.IntEnum):
    E1 = ...
    E2 = ...
    E3 = ...

E1 = E.E1
E2 = E.E2
E3 = E.E3

When a Python API maps a C/C++ API, enum members are typically declared as int. But this is a pity, because it does not catch mixing of enum types. Also, when an enum match a C enum in an API, it is better if the class representing the type, does not have any member, because the members are aliased outside. Yes, one can write all enums instances are all of the same int type, but this is braking typing.

Also, there is an inconsistency, because in a loadable module, MyPy does not complain if an enum has no member. So why not allow it in stubs?

A practical case where the idea came to me, but I was sad to discover it does not work in stubs, while MyPy did not complain when I tried an enum with no members in a loadable module: AlexionSoftware/types-wxpython#42 (comment) . I prefer to wait for comments on this proposal, before editing the message linked, to say unfortunately it does not work, would prefer saying it just requires an option to be set.

Instead of a working clean idiom, I get this kind of error for each enum class declaration:

error: Detected enum "wx.StandardPaths.ResourceCat" in a type stub with zero members. There is a chance this is due to a recent change in the semantics of enum membership. If so, use member = value to mark an enum member, instead of member: type [misc]

[Hibou57]

The proposal may or may not be accepted, so I devised another way. At least, this works with regular Python:

class Enum1(int):
    pass

E1: Enum1 = Enum1()
E2: Enum1 = Enum1()
E3: Enum1 = Enum1()

class Enum2(int):
    pass

E4: Enum2 = Enum2()
E5: Enum2 = Enum2()
E6: Enum2 = Enum2()

A: Enum1 = E1  # OK
B: Enum2 = E1  # "Incompatible types in assignment", as expected
C: int = E1  # It works, like in C++

And this, seems to be accepted by MyPy in stubs:

class Enum1(int):
    pass

E1: Enum1
E2: Enum1
E3: Enum1

Will go with it instead, it's up to the author(s) of MyPy to decide if this proposal is to be kept or dropped in favor of this alternative.

For the story, it is the first idea I had, but I was afraid it may not be enough good for Python, because it has the enum package. Finally, this solution has the same properties as the ones of the previous and even better, because it does not add anything to int and MyPy accept it even in stubs.