Formatter: Ellipses in stub files get formatted on a single line

[konstin]

If a class, method or function is stub out with an ellipsis (...), black gives them their own line in python files and appends them end-of-line after the colon in stub files:

scratch.py

class Animal:
    ...


class Cat(Animal):
    def meow(self):
        ...


def f():
    ...

scratch.pyi

class Animal: ...

class Cat(Animal):
    def meow(self): ...

def f(): ...

[MichaReiser]

Note: This requires tracking the source type and passing it to the formatter context.

[andersk]

With any luck, Black will soon be changing here to allow the concise form in both .py and .pyi files.

• Allow reduced number of blank lines for dummy implementations psf/black#1797
• More concise formatting for dummy implementations psf/black#3796

[konstin]

Applying the same formatting for source and stub files would be nice, and it would also reduce formatter complexity because it gives us one variable that changes formatting less.

[hauntsaninja]

Black still handles other things (like whitespace) differently between py and pyi, so you'll still need to put source type in formatter context if you wish to format stub files like Black does.

[MichaReiser]

We should probably implement this anyway for now, because the change will be part of Black's new preview style.

[konstin]

Added the file type information to the context #6331

[konstin]

PyFormatOptions now has a source_type field

[saulshanabrook]

Hey! I would love to try to adopt this formatting in my normal Python files as well. I saw that black I believe supports this under the next flag and was curious if that might be possible to support in ruff too.

---

For what it's worth, my use case is for a library that allows other libraries to create DSLs based off uninterpreted functions, so none of them will have bodies. I have gotten some feedback that it's annoying that it takes an extra line for every definition.

[MichaReiser]

@saulshanabrook You can give it a try by running the formatter with --preview. We plan to release the new preview styles soon. See #8678

[saulshanabrook]

Oh wow, thank you @MichaReiser! That's great!