Thanks for the heads-up @guettli, I've never taken a look at blue before.

Do you know if the internals are at all similar to black's? Darker uses the Python (non-public, see psf/black#779) API of Black instead of calling it as a subprocess.

Design

See also #304 for the idea to allow to disable reformatting.

Alternative 1: Separate --black/--blue configuration options for both formatters

Pros: Similar to the --isort option we already have.
Cons: Somewhat tricky to implement, plus the question about --no-black and --no-blue.

pyproject.toml

In absence of formatter related command line options:

[tool.darker]
black = true  # the default, uses Black as the formatter

[tool.darker]
blue = true  # disables Black and uses Blue as the formatter

[tool.darker]
black = false  # disables reformatting, see also #304

[tool.darker]
blue = false  # no effect, Black still enabled

Command line arguments

Examples in absence of pyproject.toml configuration:

$ darker .  # use Black (the default)
$ darker --black .  # no effect, still use Black
$ darker --blue .  # use Blue, disable Black
$ darker --no-black .  # disable reformatting (do we want this? see also #304)
$ darker --no-blue .  # no effect, still use Black (do we want this?)

Alternative 2: One common --formatter option

Pros: Similar to the --lint option we already have; simple to implement.
Cons: More verbose and maybe not so intuitive for the user.

pyproject.toml

In absence of formatter related command line options:

[tool.darker]
formatter = "black"  # the default, uses Black as the formatter

[tool.darker]
formatter = "blue"  # disables Black and uses Blue as the formatter

[tool.darker]
formatter = ""  # disables reformatting, see also #304
formatter = "none"  # alternative syntax, which one is better? (see #304)

Command line arguments

Examples in absence of pyproject.toml configuration:

$ darker .  # use Black (the default)
$ darker --formatter=black .  # no effect, still use Black
$ darker --formatter=blue .  # use Blue, disable Black
$ darker --formatter= .  # disable reformatting (do we want this? see #304)
$ darker --formatter=none .  # disable reformatting (alternative syntax, see #304)

@akaihola sorry. I have not looked at the internals of blue. AFAIK it is black with some small modifications.

Ok, I'll return to possible blue support after I've made the 1.4.0 release.

One tricky problem to solve is the command line option for this. I've already regretted that --black isn't an option, and running black is the default.

@guettli & @jedie, I'm now returning to this idea since 1.4.0 has been released.

The first thing I had to ponder is the command line interface and configuration options. I added a design section to my first comment above – what's your take on the two alternative configuration syntaxes, or do you have further ideas?

@akaihola I prefer Alternative2 ("formatter") a bit. Maybe in the long run there is a third formatter (maybe called "green"). I think then Alternative2 makes more sense. But no strong feelings here. A1 is fine, too.

+1 for --formatter

#309 is a placeholder pull request to start working on this.

I thought about this again. Since Django will use black, I think I will do so, too. Sometimes it is easier to drop the own opinion and go with the flow.

Thanks for the note, @guettli. I'll still keep this idea around, but deprioritize it and move it to a future milestone.