Reflows of multiline blocks that have only one value don't include a comma

[quodlibetor]

Operating system:
Python version: any
Black version: 18.6b4
Does also happen on master: yes

# formatted
var = [
    # A Thing that is very important oooooooooooooooooooo aaaaaaaaaaaaaaaaaaaa
    "VALUE"
]

I would expect that to insert a comma after "VALUE" for consistency with the expected output if there are multiple values, for example:

# example 2 formatted
var = [
    # A Thing that is very important oooooooooooooooooooo aaaaaaaaaaaaaaaaaaaa
    "VALUE",
    "OTHER",
]

[carljm]

In general Black doesn't add the trailing comma for single-element lists, but I can see how it maybe should do so when the list is exploded due to a comment. Thanks for the report!

[quodlibetor]

Yup it's a edgy edge case.

[ambv]

We will address this. It's a duplicate of #274.