Black compromises on commas in nested dict literals? 😱

D = {
    "dummy": None,
    **({"has comma": 123,} if some_global else {}),
}
D = {
    "dummy": None,
    **({"no comma?": 123} if some_global else {}),
}

Oddly black is happy with and without trailing comma in the inner dict literal. And there I thought it was an uncompromising code formatter !😉

> black t.py
All done! ✨ 🍰 ✨
1 file left unchanged.
> black --version
black, version 19.10b0

[Peque]

As @dimaqq pointed out, Black is okay with either formatting, and that should not be the case.

If I had to choose, I would prefer not having a trailing comma when the collection is on a single line (for simplicity and aesthetics).

If I had to be more more objective, then I would say that is still a better choice since it would be flake8 compatible. So why further differ from other widely used linters/formatters?

[durin42]

That kind of has two parts to it, so I’m going to write out two questions and answer them:

Q) Why isn’t the trailing comma deleted?

A) Because in order to implement #826 we had to stop deleting trailing commas. Black used to do that uncondtionally and then reinsert them if a collection was split onto multiple lines. Now it simply no longer deletes trailing commas.

Q) If that collection has a trailing comma, why isn’t it exploded onto multiple lines?

A) Because I couldn’t figure out how to make the logic work coherently for “inner” collection literals, the explode-if-trailing-comma logic only applies on the outermost collection literal.

Hopefully that clarifies? In any event I’m not a black maintainer, so I can’t speak to whether or not they consider either of these behaviors a defect.