I001: Support for magic trailing commas

[Jackenmen]

Black auto-explodes imports, collections, function calls, etc. when they contain a trailing comma:
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#the-magic-trailing-comma

This means that even though this would fit into one line:

from seven_dwwarfs import (
    Grumpy,
    Happy,
    Sleepy,
    Bashful,
    Sneezy,
    Dopey,
    Doc,
)

Black will NOT convert it into this:

from seven_dwwarfs import Grumpy, Happy, Sleepy, Bashful, Sneezy, Dopey, Doc

As long as the import list contains a trailing comma.

Ruff currently does not have the magic trailing comma handling as far as I can tell and will collapse the import list if it can fit within the line length limit.

See relevant fixed (but not released) issue in isort: PyCQA/isort#1683

[charliermarsh]

Yeah this would be nice to support.

[colin99d]

I am going to attempt this one.

[charliermarsh]

Sweet. We'll probably need some logic to detect whether an import block ends in a trailing comma, then propagate that through as we normalize and combine the import blocks in the isort module, then thread through a flag to format_import_from to take the format_multi_line path if we have a trailing comma.

[jaxwagner]

Hi! I'm encountering some strange behavior where the formatter will:

• add the comma if there is a single argument that takes up multiple lines
• remove the comma if single arg on one line
• add the comma if multiple args on multiple lines
  in particular, the first bullet seems strange to me. I have the following configs:

...
[lint.isort]
...
split-on-trailing-comma = false
...
[format]
skip-magic-trailing-comma = true

is this expected behavior? Thanks so much!

[charliermarsh]

@jaxwagner - Are you able to include a Python snippet to demonstrate the behavior you're seeing, and what you find surprising? Happy to help if so :)

[jaxwagner]

def _func(
    input_values: Sequence[
        Union[CxxxxxxxxxxxxxxxxxxxxxxT, PxxxxxxxxxxxT, CxxxxxxxxxxxxT, BcccT]
    ],
) -> Type:

here's an example where the comma was added. this only happens when there is a single argument in the function that spans multiple lines. is this expected? thanks so much @charliermarsh ! :)

in this case, there was an existing comma that was taken away:

async def cxxxxxxxxxxxxxxxxxxxxxxxxx_js(
    txxxxxxxxxxxir: Vxxxxxxxxxxxxxxxxxxxxy  # used to be a comma at end of this line
) -> bool:

[charliermarsh]

@jaxwagner - Ahh yes, that first example is intended. However, if you shorten the annotation, the formatter should re-collapse it given that you have skip-magic-trailing-comma = true set.

I can't reproduce that second example -- for me, the formatter is always adding a comma there (https://play.ruff.rs/b777c7bf-78e5-492d-aa3a-0869b930d565).

[jaxwagner]

here's an example where the comma is removed:
https://play.ruff.rs/8c27c3f5-1f36-48b5-b4d0-dac32a05ce02

i'm seeing a ton like this

[charliermarsh]

Ooh interesting, thank you! It's the trailing comment that makes the difference: https://play.ruff.rs/0c1f8900-2880-402e-a68b-0537d73e7b61

I'll file a ticket to explore this (it may be working as intended but it deserves a second look).

[jaxwagner]

thanks @charliermarsh!