Support for enum(IntFlag) in addition to enum(IntEnum)?

[nurpax]

Now that Nanobind has support for enum(IntEnum), supporting enum(IntFlag) seems like a nice addition.

Here's a motivating case for it:

class InputTextFlags(enum.IntEnum):
    NONE = 0
    CHARS_DECIMAL = 1<<0
    CHARS_HEXADECIMAL = 1<<1
    UPPER = 1<<2

a1 = InputTextFlags.CHARS_HEXADECIMAL | InputTextFlags.UPPER # this type checks as 'a' becomes of type int
a: InputTextFlags = InputTextFlags.CHARS_HEXADECIMAL | InputTextFlags.UPPER # this doesn't

The latter case unfortunately doesn't type check:

This can be fixed by typing functions differently, f.ex. by typing input flag arguments as int instead of InputTextFlags.

But Python enums support a more type safe alternative, the enum.IntFlag.

class InputTextFlags2(enum.IntFlag):
    NONE = 0
    CHARS_DECIMAL = 1 << 0
    CHARS_HEXADECIMAL = 1 << 1
    UPPER = 1 << 2

b: InputTextFlags2 = InputTextFlags2.CHARS_HEXADECIMAL | InputTextFlags2.UPPER

Perhaps there could be an enum annotation, say nb::is_intflag(), similar to the existing nb::is_arithmetic() flag to signal Nanobind to use IntFlag instead of IntEnum?

[wjakob]

I don't plan to work on this feature myself, but I am open to external contributions if they don't add too much extra complexity.