Add --skip-magic-trailing-comma

[hauntsaninja]

Lukasz indicated he'd accept such a PR in #1288 (comment)

This is a pretty straightforward combination of things from before and after #1612 and #826

Feedback I'm looking for:

1. What the best way to get the MAGIC_TRAILING_COMMA option to the places it's needed is.
   Obviously the global variable proof of concept is unmergeable, but I'm not sure what the right solution is. Passing it around seems kind of untenable because Leaf.append is used in a million places / should_split_body_explode has a daunting reverse call graph too (You could make store an attribute on Leaf to make things a little easier, but that's pretty ugly as well). Maybe a context manager to set a global current Mode?
2. How best to structure tests for this
   Black doesn't have many options, so there isn't a clear pattern for how to test these. I went with reusing expression.py, but I could do something more akin to numeric_literals_skip_underscores. Let me know!

[ambv]

You're right that using global variables won't be acceptable for this. Hiding the global variable by running a context manager is not fundamentally changing this either.

Having an attribute on Leaf is what I would look at.

[hauntsaninja]

Thanks! You'll notice from the diff that I meant to say Line, not Leaf — hope it's still acceptable :-) I also updated blackd and the relevant docs that I could find.

[graingert]

@hauntsaninja thanks for following up on this, I'd completely missed the concept ack on the other issue

[hauntsaninja]

@ambv if you have time to take a look, much obliged!

[hauntsaninja]

Just bumping this!

[JelleZijlstra]

we might need to be consistent here at some point:

(py38) mbpt-jelle:black jelle$ git grep behavior | wc -l
       7
(py38) mbpt-jelle:black jelle$ git grep behaviour | wc -l
       7

[uriva]

any idea when will this be released?

[ichard26]

@uriva unfortunately I can't give you a timeframe, but I do hope it will happen soon! Apologies for the lack of any clear messaging.

[uriva]

@ichard26 what is this blocked on? or is releasing itself a difficult process? (not demanding anything, just curious)

[ichard26]

@uriva I honestly don't know 100% why a new release is blocked, but the general lack of time and specifically the lack of availability for the ones holding the power to create a release is probably why. Maybe @cooperlees can explain in more detail.

[uriva]

who has this power?

[graingert]

@uriva you can see the list here https://pypi.org/project/black/#data

[indigoviolet]

tangential comment: I found this option --skip-trailing-magic-comma in the github documentation, but then didn't find it in the most recent release and it was mildly confusing. It might be useful to either have per-feature metadata like [Added in 20.0b2dev] etc or a develop branch.

[ichard26]

@indigoviolet you're not alone! I find it confusing too. I am planning to just move the documentation to ReadTheDocs where the stable version will be provided by default to avoid this mess in the first place (it's part of my effort to revamp the docs). Thanks for the feedback though!

[hauntsaninja]

Opened #2064 to add this to CHANGES.md

Separately, is there anything blocking a release that I could help with?

[adamjstewart]

I tried adding:

skip_magic_trailing_comma = true

to my pyproject.toml but it doesn't seem that this setting can be configured from a file. Could support for this be added?

[hauntsaninja]

Adding skip_magic_trailing_comma = true in the tool.black section in pyproject.toml definitely works for me. I'd recommend asking in the discord for help or opening an issue if you can't get it to work.