Issue with the "Magic Trailing Comma" in collections in function calls

[scolby33]

Describe the bug Trailing commas are left in collections that remain on one line in a function call
To Reproduce Steps to reproduce the behavior:

1. Take this file:

test_call(
    {"some_key": "some_value", "other_key": "other_value",}
)

This also happens with the set literal {"some_key", "some_value", "other_key", "other_value",} and the list literal ["some_key", "some_value", "other_key", "other_value",].

2. Run Black on it with these arguments black black_test.py
3. See error: the file is unchanged

Expected behavior
Based on the documentation on trailing commas, I would expect the final comma after "other_value" to be removed.

Unnecessary trailing commas are removed if an expression fits in one line.

This case doesn't match either of the stated exceptions: it is neither a tuple expression with one element nor a function signature with a *.

Based on the documentation on the magic trailing comma, I would expect the dictionary literal to be expanded to multiple lines.

Now, you can communicate that you don't want [the literal collapsed to one line] by putting a trailing comma in the collection yourself. When you do, Black will know to always explode your collection into one item per line.

Environment (please complete the following information):

• Version: 19.10b0, installed via pipx
• OS and Python version: macOS 10.15.4 with Python 3.8.2 installed via pyenv

Does this bug also happen on master? Yes: tested on the Black Playground

[ichard26]

Seems rather similar to #1169.

[scolby33]

Agreed! Please close as a dupe if appropriate. I missed that issue in my search before submitting this.

[cooperlees]

Duplicate of #1169