`check_type` does not work with Subscripted generic fields

[devashishshankar]

If I have a field with type_hint being something like List[A], and check_type=True, I would expect it to validate the field. However, the code throws the following error on class init:

TypeError: Subscripted generics cannot be used with class and instance checks

This seems to be the default behavior of isinstance when dealing with any subscripted generic field, for e.g. List[str] or Optional[str]. See related SO answer. Consequently validate also suffers with the same issue.

There is an easy workaround, setting check_type=False, and adding a custom validator:

f = field(type_hint=List[A], check_type=False,
              validators=lambda l: isinstance(l, list) and all([isinstance(e, A) for e in l]),)

However, I still thought I would add this issue in case someone runs into it. (Don't know if this should be handled at a library level, given this is a limitation of the default isinstance)

[smarie]

As mentioned in this doc section, pyfields relies on whatever type checking library is available on your machine.

So if you have typeguard or pytypes installed, it should work correctly.

[devashishshankar]

Ah, thanks! typeguard works but is python3 only. And pytypes doesn't work for py3.7. But these are already open issues in those libraries.