Formatter undocumented deviation: Formatting of long lambda as keyword argument

[henribru]

Black formatting:

def a():
    return b(
        c,
        d,
        e,
        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
            *args, **kwargs
        ),
    )

Ruff formatting:

def a():
    return b(
        c,
        d,
        e,
        f=lambda self,
        *args,
        **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs),
    )

In addition to being incompatible, this formatting seems very strange and not very readable, as the *args and **kwargs suddenly look like they are arguments to b.

[charliermarsh]

Thanks!

[henribru]

This seems possibly related

https://black.vercel.app/?version=stable&state=_Td6WFoAAATm1rRGAgAhARYAAAB0L-Wj4ADqAHRdAD2IimZxl1N_WlXnON2nzNX9HFvHx6cBUQ15-GcjjCXTWY4lAb3LPNtN0kban15sEExCiEMHC1fqLMN_I2mNYrKs6DX7rL7K_IqhZ4f1qfF_pwoKJfLbTWpcKTtsPNouVDJpGY10D5nlGuf_NfXkJrKjOtjHACUnbTAaUKR-AAGQAesBAAAgR-_jscRn-wIAAAAABFla

https://play.ruff.rs/c9b4e3fa-4401-4248-a98d-d9ae6b984697?secondary=Format

[charliermarsh]

Yeah, looks like we have some refinements to make in the lambda formatting. On it! (Thanks for sharing playground links, it helps a lot.)

[DarthMuzammil]

@charliermarsh Can i take this issue up?

[MichaReiser]

@DarthMuzammil sure. Let me know if you need any help

[charliermarsh]

@DarthMuzammil -- have you had a chance to look into this? If not, do you still plan to? (No pressure, I just want to make sure we find an owner soon if it's not being addressed.)

[MichaReiser]

It seems black never inserts soft line breaks between parameters. This would be an easy fix... except for comments

Although Black's formatting also has its shortcomings when you have default values:

def a():
    return b(
        c,
        d,
        e,
        f=lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
            *args, **kwargs
        ), e=1, f=2, g=2: d,
    )

It could be nice to simply parenthesize the lambda expression and indent it by 1 level (and otherwise keep our formatting). However, that would also mean that the formatter must remove the parentheses again when the lambda fits on a single line—something we haven't really done before in sub-expression positions. And there's also no real good place to add it, because this should be done when formatting lambas in any position.

[DarthMuzammil]

@charliermarsh Hey, sorry for the delay i just saw this message, yes i will start working on this today

[DarthMuzammil]

It seems black never inserts soft line breaks between parameters. This would be an easy fix... except for comments

Although Black's formatting also has its shortcomings when you have default values:

def a():
    return b(
        c,
        d,
        e,
        f=lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
            *args, **kwargs
        ), e=1, f=2, g=2: d,
    )

It could be nice to simply parenthesize the lambda expression and indent it by 1 level (and otherwise keep our formatting). However, that would also mean that the formatter must remove the parentheses again when the lambda fits on a single line—something we haven't really done before in sub-expression positions. And there's also no real good place to add it, because this should be done when formatting lambas in any position.

@MichaReiser How should i go about solving this issue?

[MichaReiser]

@DarthMuzammil not sure. I think coming up with a style that works with the presence of comments (which black does not always) is the main work of this issue. My preferred solution would be to just parenthesize the lambda parameters if it breaks, but Python3 removed support for parenthesized lambda parameters for some reason :(

# Using parentheses is a syntax error
def a():
    return b(
        c,
        d,
        e,
        f=lambda (
            self, 
            araa, 
            kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
                *args, **kwargs
            ), 
            e=1, 
            f=2, 
            g=2
        ): d,
    )

Related black issue: psf/black#809