Trailing comma on single-line nested dict

[NakulK48]

Describe the bug A clear and concise description of what the bug is.

If a dict is nested within another dict, it doesn't reformat correctly.

To Reproduce Steps to reproduce the behavior:

Take this:

my_dict = {
    "outer": {
        "a": 1,
        "b": 2,
    }
}

Pass it through black:

my_dict = {"outer": {"a": 1, "b": 2,}}

This has a trailing comma, which violates https://www.flake8rules.com/rules/E231.html

Expected behavior A clear and concise description of what you expected to happen.

Either the snippet unchanged (which is what happens when the dictionary isn't nested)

my_dict = {
    "a": 1,
    "b": 2,
}

Or a single-line dict with no trailing comma (black on this returns with no change)

my_dict = {"outer": {"a": 1, "b": 2}}

Environment (please complete the following information):

• Version: 19.10b0
• OS and Python version: Ubuntu 18.04, python3.8.2

Does this bug also happen on master?

Yes

Additional context Add any other context about the problem here.

[NakulK48]

I'll add as a stylistic note that it's kind of nice to be able to enforce dict fields going on new lines without having to do fmt: off

[ichard26]

This is part of #1288. The 'Magic Trailing Comma' doesn't work on nested collections yet.

However, there's issues with it so far. It doesn't yet work on nested collections. Worse yet, in those cases it leaves trailing commas behind when a collection ends up compressed into one line. We need to fix those edge cases to make it generally dependable.

[ichard26]

Fixed by #1612 which allows the trailing comma to work on nested collections.

(black-rHKUX7ap) R:\Programming\black>type temp.py
my_dict = {
    "outer": {
        "a": 1,
        "b": 2,
    },
}

(black-rHKUX7ap) R:\Programming\black>black temp.py --diff --color
All done! ✨ 🍰 ✨
1 file would be left unchanged.

Environment:

• Black version: Black 20.08b1
• Python version: CPython 3.8.1
• OS version: Windows 10 Home Edition Build 18363