Replace Union annotations with TypeVar

[WillAyd]

Had a revelation after python/mypy#6847 (comment) and I think we've been using Union too liberally in places where a TypeVar may make more sense (especially in pandas._typing) so would be good to go back and swap those out.

@gwrome @vaibhavhrt

[vaibhavhrt]

@WillAyd we are almost done with removing all ignored modules from mypy.ini Once that's done I will take a look where we have used Union and see if we can use TypeVar instead.

[vaibhavhrt]

@WillAyd I will look for Union in pandas._typing and after fixing that will look for Union in other files.
Just to be sure typeVar for Union[str, float] would be Something = TypeVar('Something', str, float), right?
Also can please provide some explanation on when to use TypeVar and when to use Union.

[WillAyd]

TypeVar is going to be more suitable for parametrization and generic functions. In the case of containers I think the difference between:

T = TypeVar('T', str, float)
List[T]

and

List[Union[str, float]]

Would be that the former could only accept [1, 2] and ['a', 'b'] but not [1, 'a']. The latter would accept all of those.

Looking at functions:

def foo(something: T) -> T:
    ...

Would basically mean that if foo accepts a string it also returns a string, and if it accepts a float it returns a float, but that it cannot accept a string and return a float. By contrast:

def foo(Union[str, float]) -> Union[str, float]:
    ...

Would allow all of those combinations, so it could accept 'a' and return 1.

We've used Union throughout so far but more often than not I think parametrization of the Type is actually what we want.

[vaibhavhrt]

Nice, thanks for explaining, I will replace all unions in pandas._typing today.

[jreback]

does #26588 fully close this?

[WillAyd]

Yep