Please manage commas

I expect yapf, as a style-normalizer, to handle the trailing comma issue for me. Some writers put it always, others never, and yapf should make the code look the same regardless of who wrote it.

a_very_long_function_name(
    long_argument_name_1=1,
    long_argument_name_2=2,
    long_argument_name_3=3,
    long_argument_name_4=4,
)

a_very_long_function_name(long_argument_name_1=1,
                          long_argument_name_2=2,
                          long_argument_name_3=3,
                          long_argument_name_4=4)

Desired output: (in all three cases)

a_very_long_function_name(
    long_argument_name_1=1,
    long_argument_name_2=2,
    long_argument_name_3=3,
    long_argument_name_4=4,
)

a_very_long_function_name(
    long_argument_name_1=1,
    long_argument_name_2=2,
    long_argument_name_3=3,
    long_argument_name_4=4,
)

Actual output: (note trailing comma missing in second case)

$ yapf --style facebook ./demo.py
a_very_long_function_name(
    long_argument_name_1=1,
    long_argument_name_2=2,
    long_argument_name_3=3,
    long_argument_name_4=4,
)

a_very_long_function_name(
    long_argument_name_1=1,
    long_argument_name_2=2,
    long_argument_name_3=3,
    long_argument_name_4=4
)

In this case I have to hunt down missing trailing commas and fix them by hand, which is very similar to the problem yapf was made to solve – often in code reviews you’ll see junior devs told “trailing commas are nice”.

[asottile]

for those looking for a battle-tested tool which modifies the token stream to add commas (with options for different target python versions) I’ve written add-trailing-comma

[bwendling]

In general, I’m against modifying the token stream. I did it before and it’s fraught with problems. If someone wants to submit a patch, I’ll consider it. But it’ll have to be contained behind a style knob and come with a bevy of tests…