dataclasses.Field.default and .default_factory can be _MISSING_TYPE

[altendky]

No description provided.

[srittau]

Thanks, but please use the new union syntax (i.e. Foo | Bar).

[hauntsaninja]

I considered doing this earlier, but didn't because I think this will be slightly annoying, since the idiomatic way to narrow is if x is dataclasses.MISSING, which type checkers will not understand. mypy primer has an example, you can see that this re-introduces a type error in pydantic (that was previously fixed by #5718).

There probably isn't a good solution here, but just wanted to call it out.

[JelleZijlstra]

Yes, I had the same concern as Shantanu. I think PEP 661 (https://www.python.org/dev/peps/pep-0661/) would provide a cleaner solution.

[srittau]

Maybe @altendky has a use case that requires this change? We can always change over to PEP 661 when that gets accepted and is available for use. That said, I have no opinion, what solution is best.

[Akuli]

We could define _MISSING_TYPE as an enum containing a single member, named _MISSING_TYPE.MISSING for example, and then use Literal[_MISSING_TYPE.MISSING] instead of _MISSING_TYPE. Mypy seems to support is for enums, and unlike most other things, enums can go inside a Literal.

Edit: PEP 661 mentions this as a rejected idea, but unlike most of their other rejected ideas, it actually works and it was apparently rejected only because nobody likes it.

[altendky]

Welp, of course the time I don't chat about a change first it ends up deserving discussion. :] Here's the code that made me take a look at this.

https://github.com/python-desert/desert/blob/483734f89528fef86c0f4bbd38b4ba3473acae9d/src/desert/_make.py#L324

        if field.default_factory != dataclasses.MISSING:  # type: ignore[misc]

error: Non-overlapping equality check (left operand type: "Callable[[], object]", right operand type: "_MISSING_TYPE")  [comparison-overlap]

Here's a minimal example. Note that the first error seems to be python/mypy#10750 and I was already avoiding it with # type: ignore[misc].

https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=f66bb887158648e99b97c042165dd684

import dataclasses


@dataclasses.dataclass
class C:
    f: int = 0


for field in dataclasses.fields(C):
    if field.default_factory != dataclasses.MISSING:
        pass

main.py:10: error: Attribute function "default_factory" with type "Callable[[], _T]" does not accept self argument
main.py:10: error: Non-overlapping equality check (left operand type: "Callable[[], Any]", right operand type: "_MISSING_TYPE")
Found 2 errors in 1 file (checked 1 source file)

I just tried is and is not and they both seemed to result in the same error.

Regardless, I'll be adding an ignore "for now" since of course this wouldn't be released anyways. If there's another more proper solution and this isn't a useful interrim change, I can handle that.

Thanks to all for the interest and effort here.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pydantic (https://github.com/samuelcolvin/pydantic.git)
- pydantic/dataclasses.py:174: error: unused "type: ignore" comment

[srittau]

This looks fine to me, but I'll defer judgement to @hauntsaninja and @JelleZijlstra.

[JelleZijlstra]

I'd prefer to stick with the current stub until PEP 661 gives us better tools to deal with this situation.