Skip trailing comma by default

[smurfix]

Bug description

Changing line lengths is not reversible because Black adds a trailing comma to some argument lists.

To Reproduce

def some_fn(*args):
    pass


some_fn(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)

Reformatting with --line-len=20 results in

def some_fn(*args):
    pass


some_fn(
    1,
    2,
    3,
    ...
   20,
)

Unfortunately, due to the trailing comma after the 20 that wasn't there before, re-black-ing with --line-length=80 doesn't undo this transformation.

I have to manually remove the comma to get the old formatting back.

Expected behavior

Changing the line length from X to Y and back to X should be a no-op, at least for reasonable values of X and Y.

[Jackenmen]

This handling of trailing commas is intentional and this behavior is called "magic trailing comma", you can read about it here:
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#the-magic-trailing-comma

[smurfix]

This handling of trailing commas is intentional and this behavior is called "magic trailing comma",

@jack1142 Yes I know that Black doesn't reformat lines when there's a trailing comma. That doesn't mean that it's a good idea to add a comma when there wasn't one, esp. without an option to disable that behavior.

[TomFryers]

I think this is a duplicate of #1742. The --skip-magic-trailing-comma/-C flag is the best solution right now.

[felix-hilden]

I'd be willing to entertain us flipping the toggle in preview and eventually altogether in v23. But this is just me personally 😅 Not collapsing small collections is more irritating to me than slightly larger diffs, although the config option is an easy fix. This would get us a step closer to our whole "not taking previous formatting into account" philosophy. Thoughts by other maintainers?

[TomFryers]

I think its new title makes it a duplicate of #2135 instead! That one's accumulated a few thumbs-ups, so it seems there is at least some support for this amongst users (including me). (Not that that means particularly much; it's a rather biased sample.)

[felix-hilden]

Oh good find, I'll close this and post my message there as well 👍