from_type raises TypeError for Union of builtin generic types

[ghost]

On the current version of hypothesis (6.17.4) on python 3.9.2

st.from_type(Union[list[str], str]).example()

raises TypeError("issubclass() arg 1 must be a class") because list[int] is an instance of types.GenericAlias and not an instance of type.

[ghost]

I think this guard against types that are not really types

should be

if isinstance(t, types.GenericAlias):
    ....

Bonus fact: if this needs to guard against Union too the check should also include typing._UnionGenericAlias as that is not a subclass of GenericAlias for some reason.

[Zac-HD]

Huh, yeah - thanks for the report! Looks like we missed/under-tested the equivalent of #2979 for Python 3.9 (PEP 585).

[Zac-HD]

OK, that specific check can just use a try/except TypeError.

The underlying problem is that we have never really tested Hypothesis with the new-in-3.9 types.GenericAlias type, which I expect to see a lot more of over the next few years (e.g. list[int] is a lot more attractive than constantly importing stuff from the typing module). I expect that we'll need to update a few things to check "is this type generic" in a more, um, generic way; at least accessing __origin__ and __args__ seems to work the same way.