[Reverted] Make all single-constraint TypeVars to use bounds

[Michael0x2a]

According to the documentation in the typing module, TypeVars cannot have only a single constraint. Attempting to do so will actually result in an exception at runtime. (However, this error is currently ignored by mypy -- see python/mypy#2626 for a related pending pull request).

This commit changes all instances of TypeVars using a single constraint (e.g. T = TypeVar('T', Foo)) to use bounds instead (e.g. T = TypeVar('T', bound=Foo).

This seems to be the correct fix for plistlib after reading the module docs, but it's less obvious this is correct for unittest. The unittest module originally had _FT = TypeVar('_FT', Callable[[Any], Any]) -- an alternative fix would have been to do _FT = Callable[[Any], Any].

Although I'm not entirely sure what it means to have a bound be a Callable, I decided to make the assumption that the original authors probably meant to use TypeVars instead of type aliases for a reason (possibly to handle classes implementing __call__?)

[ambv]

Also unsure about the unittest case but we can fix it forward if some edge reveals we should have gone with _FT = Callable[[Any], Any] instead.

[gvanrossum]

Now that I've read this over carefully, I unfortunately think that the unittest fix is incorrect. I have this simple test:

from unittest import skip, TestCase

class TC(TestCase):
  @skip("not today")
  def testFoo(self) -> None:
    assert False

Before this PR, that works, but with the change to unittest/__init__.pyi, it gives an error on the @skip line:

__tmp__.py:4: error: Argument 1 has incompatible type Callable[[TC], None]; expected None

I believe the problem is that you currently can't return a generic type from a function without generic arguments. Apparently the old form worked around this (perhaps it just removed the genericity from the return type, which makes sense as an edge case of a TypeVar with value constraints). There's an open issue for this: python/mypy#1551

So sadly I'm going to revert this -- we first need to fix the latter issue. :-(

[Michael0x2a]

Bleh, I figured there was something odd afoot. I'm not at a computer right now, but I can poke around/try and find a workaround sometime later today or tomorrow.

[Michael0x2a]

@gvanrossum -- I'm slowly poking around with the TypeVar stuff, but in the interim, do you think using _FT = Callable[..., Any] would be a good workaround? (I don't think _FT = Callable[[Any], Any] actually works since then you couldn't add @skip or similar annotations to top-level functions that take in no arguments).

[gvanrossum]

Yeah, I think _FT = Callable[..., Any] should be fine, as long as we add a TODO comment showing the correct TypeVar definition and a mention of the issue it's waiting for.

[gvanrossum]

I'm going to attempt a new band-aid fix so we can finally move forward with #2626.