multimethod dispatch on typing.Literal

[adam-urbanczyk]

Would it be possible to implement multimethods dispatching on typing.Literal?

[coady]

I don't think so. The dispatch - and moreover the caching - relies on issubclass. Anything that would make the dispatch as slow as a linear scan would probably be better as part of overload, which already does that. It only supports predicate functions at the moment, but that could be extended.

Another workaround is to use an enum.

[adam-urbanczyk]

Thanks, I'm aware that enum would work but it is not a part of the current API of the module I'm trying to use multimethod with. Would it (almost) work with a custom Literal type implementing __subclasshook__? I assume that you could cache the resolutions based on the args of the Literal - for all practical uses I have seen either str or numbers.

FYI: what I would like to achieve in the end is dispatching based on typing annotations including Literal.

[coady]

Would it work for your use case to partially support Literal, i.e., still be subclass based.

Literal['a', 'b']) -> str
Literal['a', 0]) -> Union[str, int]
Literal['a', None]) -> Optional[str]

It wouldn't check the values, but it would allow a Literal annotation to be present, with a sanity check.

[adam-urbanczyk]

I think that would work! I have this kind of signatures:

 def constrain(self, tag: str, constraint: Literal['a','b','c'], arg: Any)

and

 def constrain(self, tag1: str, tag2: str, constraint: Literal['a','b','c'], arg: Any)

[adam-urbanczyk]

Thanks @coady !