Single-argument function includes trailing comma inconsistently

[charliermarsh]

Describe the bug

When a single-argument function definition breaks across multiple lines, Black will add a trailing comma (after the argument) in some cases but not others.

To Reproduce

For example, given the following Black adds a trailing comma after the annotation on x in the first case, but not the second, third, or fourth:

# Black inserts a trailing comma after the first argument's annotation here.
def long_function_name_goes_here(
    x: Callable[List[int]]
) -> Union[List[int], float, str, bytes, Tuple[int]]:
    pass

# But not here, or in any of the below.
def long_function_name_goes_here(
    x: Callable[[str, Any], int]
) -> Union[List[int], float, str, bytes, Tuple[int]]:
    pass

def long_function_name_goes_here(
    x: Callable[List[int, str]]
) -> Union[List[int], float, str, bytes, Tuple[int]]:
    pass

def long_function_name_goes_here(
    x: Callable[List[int], str]
) -> Union[List[int], float, str, bytes, Tuple[int]]:
    pass

So the inclusion of the trailing comma seems to depend on the content of the annotation (but not its length). One observation: Black omits the trailing comma when the annotation itself contains a comma.

(Here's the same example in the playground.)

[JelleZijlstra]

Thanks. I think we should always add a trailing comma here.

[drhagen]

I would argue that Black preserving the trailing comma would be more consistent with the overall behavior of Black. For example, both of these function calls are acceptable to Black:

# Preserves lack of trailing comma
really_really_really_really_really_long_name(
    really_really_really_really_really_long_name
)

# Preserves trailing comma
really_really_really_really_really_long_name(
    really_really_really_really_really_long_name,
)

The reason for this is that lists of things is one of the places that Black gives users a choice of formatting: (1) I want as few of lines as possible, (2) one line per item. This creates an ambiguity when there is only one element: Did the user intend to choose the everything-on-one-line option (no comma) or did the user intend to choose the split-elements-across-lines option (trailing comma)? Everywhere else, Black resolves this ambiguity by preserving trailing commas, and I think it should do that in function definitions too, even single parameter functions.

[JelleZijlstra]

@drhagen possibly, but we already add the trailing comma for non-annotated parameters, and if nothing else we should be consistent.

% black  -l 20 -c 'def some_call(aaaaaarg): pass'
def some_call(
    aaaaaarg,
):
    pass