TYP: Mypy workaround for NoDefault

[twoertwein]

This uses the mypy-workaround from https://github.com/python/typeshed/pull/7127/files for NoDefault (works also for pyright).

no_default is public but NoDefault is luckily not public, so it should be fine to do the following:

• rename the existing NoDefault to _NoDefault (implementation and typing)
• define NoDefault as NoDefault = Literal[_NoDefault.no_default] (implementation and typing; isinstance checks need to be re-written to use is no_default but all other type annotations can stay the same)
• no_default "remains" no_default = _NoDefault.no_default (just replacing NoDefault with _NoDefault)

@simonjayhawkins @jreback @Dr-Irv

edit:

Some context: The following will work without mypy errors afterwards:

def foo(x: NoDefault | int) -> int:
    if x is no_default:
        return -1
    return x + 1

[twoertwein]

Something similar could also be done for NaT and NaTType

[twoertwein]

all the closed annotations seem unrelated but they are a result of this line

[twoertwein]

Instead of defining NoDefault in lib.pyx and lib.pyi it could be defined in _typing.py (would require changing a lot of import statements)

[jreback]

its actually better where it is as we avoid circular deps this way

[Dr-Irv]

Two comments:

1. There is a PEP for sentinel values: https://peps.python.org/pep-0661/ so if that PEP is approved, maybe we should use a pattern here that makes it easy for us to switch to using that feature if it gets approved.
2. There is a lot of usage of Literal["left", "right"] in this PR, so maybe we should create a type for that in _typing.py and use that instead.

[twoertwein]

1. There is a PEP for sentinel values:  https://peps.python.org/pep-0661/ so if that PEP is approved, maybe we should use a pattern here that makes it easy for us to switch to using that feature if it gets approved.

Once we can use this PEP (might take a long time, especially without typing_extensions), we simply need to change the two lines that define NoDefault and no_default on top of this PR (the PEP also encourages is for comparison, not isinstance). Technically, we would then need to change no_default when switching to this PEP which would technically break the API as it is a public symbol.

2. There is a lot of usage of `Literal["left", "right"]` in this PR,  so maybe we should create a type for that in `_typing.py` and use that instead.

Will do

[Dr-Irv]

Once we can use this PEP (might take a long time, especially without typing_extensions), we simply need to change the two lines that define NoDefault and no_default on top of this PR (the PEP also encourages is for comparison, not isinstance). Technically, we would then need to change no_default when switching to this PEP which would technically break the API as it is a public symbol.

OK, I see the path forward. However, while no_default is a public symbol, we don't document what it does, so I don't think changing it to be a sentinel will break anything. In any case, that is something that wouldn't need to be done for a few years.

[Dr-Irv]

I'm fine with this, although I think others should approve as well.

[jreback]

lgtm

[mroeschke]

Thanks @twoertwein