Fix deprecated UnicodeDelimitedTuple due to deprecated _field_types

[NazarioJL]

This addresses

• Python 3.9 deprecates _field_types from NamedTuple #32

The _field_types was removed in Python 3.9 causing UnicodeDelimitedTupleAttribute to not function properly.

This PR needs to be rebased onto #34

[NazarioJL]

@ikonst 👀

[ikonst]

Are you trying to parse Union types here?

[NazarioJL]

Yes. This works for both, 3.8 and 3.9. In 3.9 Optional[T] types are used as Union[NoneType, T], but __args__ has the specific types in either. It will basically find the first parsable string and assign that value. There are inherent issues (that already existed) for example:

class AmbivalentTuple(NamedTuple):
    a_value: Union[str, int, float]
    b_value: Union[int, str, float]
    c_value: Union[float, int, str]

If the persisted record looks like "2::2::2" we will get a tuple that looks like:

AmbivalentTuple(a_value="2", b_value=2, c_value=2.0)

[ikonst]

I think __args__ exist in any generic type, not just Union, so at the very least I'd ensure the type is Union. But honestly I'd omit this. The user would get an TypeError: Cannot instantiate typing.Union error and that'll be enough to understand that the NamedTuple should consist of simple types.

[NazarioJL]

So leave as is?
In this case:

>>> typing.Union[int, str].__args__
(<class 'int'>, <class 'str'>)

[ikonst]

Yeah, don't try to parse __args__. Simply t(value) and be done with it (like we did before).

[NazarioJL]

Sounds good, I think It would be nicer to do so, I'll go ahead and remove this.

[ikonst]

Constructs like typing.Union within the Python ecosystem are intended for type checkers to parse, not runtime. I doubt there's many counter-examples, perhaps in specialized libraries like pydantic? A DB ORM making any sense of them is probably an example of a library doing things you don't expect it to :)

One possibility could be to even do away with the option of parsing into an int. In that case, you can forego all this annotations inspection and define:

T = TypeVar("T", bound=Tuple[str, ...])

and only do return self.tuple_type(*values) in the deserializer.

However, this runs counter to the sample code. :)

[NazarioJL]

@ikonst Removed!

[matthewpick]

Thanks @mathom for originally finding this issue during #31