Unnecessary trailing commas are no longer removed if an expression fits in one line.

[maximdeclercq]

Unnecessary trailing commas should be removed if an expression fits in one line.
This doesn't happen for multiline lists e.g.

To Reproduce:

1. Run Black on the following code without arguments

test = ["a very short line", "another element", "something long", "lorem ipsum dolor sit amet"]

2. Black produces the following output as expected

test = [
    "a very short line",
    "another element",
    "something long",
    "lorem ipsum dolor sit amet",
]

3. Change the last element to "lorem ipsum", and run Black on it without arguments.
4. Black changes nothing

test = [
    "a very short line",
    "another element",
    "something long",
    "lorem ipsum",
]

Expected behavior
Running Black on

test = [
    "a very short line",
    "another element",
    "something long",
    "lorem ipsum",
]

Should have the same result as running Black on

test = [
    "a very short line",
    "another element",
    "something long",
    "lorem ipsum"
]

Output:

test = ["a very short line", "another element", "something long", "lorem ipsum"]

Environment:

• Version: 20.8b1
• OS and Python version: Linux/Python 3.8.5

Does this bug also happen on master?
Yes, see this link.

[ichard26]

Black used to remove the trailing comma if the expression fits in a single line, but this was changed by #826 and #1288. Now a trailing comma tells Black to always explode the expression. This change was made mostly for the cases where you know a collection or whatever will grow in the future. Having it always exploded as one element per line reduces diff noise when adding elements. Before the "magic trailing comma" feature, you couldn't anticipate a collection's growth reliably since collections that fitted in one line were ruthlessly collapsed regardless of your intentions. One of Black's goals is reducing diff noise, so this was a good pragmatic change.

So no, this is not a bug, but an intended feature. The reason why you're filing this issue is probably since we say the following in the (outdated) style documentation:

Unnecessary trailing commas are removed if an expression fits in one line. This makes it 1% more likely that your line won't exceed the allotted line length limit. Moreover, in this scenario, if you added another argument to your call, you'd probably fit it in the same line anyway. That doesn't make diffs any larger.

We missed that this paragraph became incorrect when the "magic trailing comma" feature was introduced. It was eventually fixed in commit 6b935a3, but that was after the stable documentation was released alongside Black 20.8b1.

Anyway, here's the documentation on the "magic trailing comma". Hopefully that helps and sorry for the possible confusion.

[zzzeek]

it would be super awesome if this were to be the second thing that black allows to be configurable. because right now it's arbirary, if I have code like:

def x():
    foo = ('one', 'two', 'three')

    bar = ('one', 'two', 'three',)

now it becomes:

def x():
    foo = ("one", "two", "three")

    bar = (
        "one",
        "two",
        "three",
    )

that is terrible. edit: obviously we're just going to do the black thing and leave it but this seems prickly enough to have an option.