An issue with forward declarations and recursive type

Example:

import typing

from pytypes import type_util

Container = typing.Union[
    typing.List['Data'],
]

Data = typing.Union[
    Container,
    str, bytes, bool, float, int, dict,
]

type_util._issubclass(typing.List[float], Container)

Traceback (most recent call last):
  File "<redacted>/lib/python3.6/site-packages/pytypes/type_util.py", line 1387, in _issubclass_2
    return issubclass(subclass, superclass)
TypeError: Forward references cannot be used with issubclass().

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 14, in <module>
    type_util._issubclass(typing.List[float], Container)
TypeError: Invalid type declaration: float, _ForwardRef('Data')

[Stewori]

I think Union + forward declarations.

I mean are there examples without Union, i.e. is this issue specific to Union checking or do we have to think more general?

Are you sure? Keeping track might introduce additional performance cost. But from design perspective this is probably cleaner, yes.

My philosophy in pytypes is to allow user to opt out of such arguable stuff. Aside that I value correctness higher than performance (in production, one would disable typechecking anyway I suppose). Also, that’s why I think of a fast-succeed mode.

[mitar]

You are amazing! I will check it immediately!