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pythonpython-typingduck-typing

How to annotate a type that is a combination of multiple duck-types?

Let's say I want to annotate the parameter to a function, and it should satisfy both typing.Sized and typing.Hashable (any other 2+ types could apply, I just picked these two for the sake of example). How would I annotate this?

Given that one can combine types as an "or" using Sized | Hashable, I would expect something like: Sized & Hashable or Sized + Hashable to work, but unfortunately it doesn't (at the time of writing on Python 3.10).

Is this supported at all, and if so with what syntax?

Solution

  • Sized & Hashable is a reasonable guess, but unfortunately intersection types are not supported.

    You'll have to define your own subclass that inherits from both:

    class SizedHashable(Sized, Hashable):
    pass

    There seems to be some effort being made to support intersection types, but I can't find any updates beyond this brief mention.