Add support for optional trailing comma

[17Reset]

When I format my code using ruff format, after the format, there is always a comma after the code, can this be avoided by a parameter? For example, my source code:

FILE_SUFFIX_ICON = {
    MATLAB_SUFFIX: MATLAB_ICON,
    CPP_SUFFIX: CPP_ICON,
    SV_SUFFIX: RTL_ICON,
    'Default': FILE_ICON
}

After using format:

FILE_SUFFIX_ICON = {
    MATLAB_SUFFIX: MATLAB_ICON,
    CPP_SUFFIX: CPP_ICON,
    SV_SUFFIX: RTL_ICON,
    'Default': FILE_ICON,
}

[tjkuson]

The Ruff formatter tries to follow the Black code style, which adds a trailing comma. A configuration option has not been added to Ruff to change that behaviour (it would have to be a new feature).

Related discussion: #9350

[17Reset]

Thanks, hope to add this feature option.

[MichaReiser]

There's no such option today. Would you mind explaining your motivation for omitting the trailing comma?

Ruff supports the --skip-magic-trailing-comma option. It doesn't configure whether the trailing comma should be added, but it removes the trailing comma (and flattens the entries in the above example), if they all fit on a single line, e.g. after deleting or changing an entry.

[heiner]

Here's a reason this might be useful: It turns ruff format into a quite decent json formatting engine. E.g.,

cat file.json | ruff format - --isolated --config indent-width=2

However, sadly trailing commas aren't legal JSON.

[MichaReiser]

You may want to give Biome a try if you are looking for a Rust based JSON formatter.

[MichaReiser]

I'm closing this as not planned. Feel free to open a new issue to discuss the style with an outline for the use case and benefit of omitting trailing commas.

[17Reset]

It's not a popular style for trailing commas to appear after line breaks in Python lists, is it?

[heiner]

You may want to give Biome a try if you are looking for a Rust based JSON formatter.

That's less useful as a quick-and-dirty way to format JSON within Python code, where the existence of ruff is more or less guaranteed.

Still, ack'd on this being a bit fringe.

[mshahbazi]

The issue arises when the formatter adds a trailing comma, which prevents the code from being one-liner, when shortened, due to the presence of the comma, unless the trailing comma is removed manually.

[nyssance]

Black style for trailing comma is stupid.