Add option for printing a colored diff

[dougthor42]

Fixes #994.

This adds a --color option to the black command line which adds very basic color support when printing diffs.

• The coloring mimics git diff because why not. Headers are Bold White, line markers Cyan, subtractions Red, and additions Green.
• There is no change to the default behavior.
• --color has no effect if --diff is not supplied.

Example:

Discussion

• As is, this will not work with Windows CMD or PowerShell. In order to enable Windows support, the colorama package should be included. I'm not sure how you feel about adding packages all willy-nilly, so I figured the first version of this PR would not include it.
• I opted to add COLOR_DIFF to WriteBack because that meant I didn't have to change any call signatures.
• An initial version used an enum for --color which let the user choose between always, none, and auto. This is still doable, but will take more effort. Personally I don't much see the need for it - using a boolean is sufficient in my opinion. That said, if you'd like me to pursue it just let me know.

[dougthor42]

Whoops, wasn't working off the latest master. Fixed and rebased.

[zsol]

FWIW I like this feature. How about dropping the command line flag and always outputting colored diff if we're writing to a terminal (sys.stdout.isatty())?

As for Windows support, I'd be fine with adding colorama as an extra (extras_require in setup.py)

[dougthor42]

dropping the command line flag

I think it would be prudent to keep some CLI flag because I'm sure there are cases where a user wants to write to a terminal but not colorize it. Perhaps we just invert it: --no-color.

if we're writing to a terminal (sys.stdout.isatty())

From what I can tell, whenever WriteBack.DIFF is used, black is already writing to io.TextIOWrapper(sys.stdout.buffer, ...). Do I even need to check isatty()?

[dougthor42]

I don't like this, but I don't fully understand what detach does or why it's needed, so this is the best I could come up with.

[zsol]

lgtm. I'll add a changelog entry and merge this soon. Thanks for contributing!

[jshin49]

Seems this PR is closed and merged into master. Yet, when I try to use the --color option, I get an error saying there is no such option. The version I am using is 19.10b0 on python 3.7.7 and python 3.6.10

[hugovk]

Yep, it's not yet been released:

This was merged into master in May 2020.

Black 19.10b0 was released in October 2019 (19.10 is yy.mm, see https://calver.org/).

[jshin49]

Ah I see. When is the next release date?

…

On Thu, 18 Jun 2020 at 21:42, Hugo van Kemenade ***@***.***> wrote: Yep, it's not yet been released: This was merged into master in May 2020. Black 19.10b0 was released in October 2019 (19.10 is yy.mm, see https://calver.org/). — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#1266 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACFDBU2KZYGPRHBVJZGOPULRXIDS7ANCNFSM4KRUYVJQ> .

[hugovk]

I think it's coming sooner rather than later!

#517 is the tracking issue, and see #517 (comment) and https://github.com/psf/black/labels/stable.

[cooperlees]

GitHub master or even a branch is basically released these days. Introducing your new friend if you're in a hurry for color:

pip install git+https://github.com/psf/black.git@master

cooper-mbp1:~ cooper$ /tmp/tb/bin/black --help | grep color
  --color / --no-color            Show colored diff. Only applies when

[jshin49]

Awesome. Thanks for the help @cooperlees. I'm trying to set up a workflow for a new team project and I was comparing between Pylint vs Black vs Yapf. Not sure which is the best, but the color coded diffs of Black would definitely make a lot of difference.