split before binary operators

[anntzer]

PEP8 recently switched to recommending splitting lines before binary ops rather than after (https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator). There are some specialized knobs available (SPLIT_BEFORE_BITWISE_OPERATOR, SPLIT_BEFORE_LOGICAL_OPERATOR), but a general SPLIT_BEFORE_BINARY_OPERATOR would be simpler to work with.

[bwendling]

Things get a bit difficult when multiple knobs address the same thing. For example, if SPLIT_BEFORE_BINARY_OPERATOR is True, but SPLIT_BEFORE_BITWISE_OPERATOR is False, what would the outcome be? We could of course specify the specific effect it will have, but then for someone to change the default behavior, they'll have to mess with the other knobs.

So I think that there should be an "all or nothing" knob. Either we have the current two knobs or we have one knob that effects both bitwise and logical operators.

[w0rp]

@gwelymernans You could have yapf exit with an error if two conflicting options are set, and explain why.

I would to see an option like this. I have actually been formatting code this way for a long time, for the reasons Knuth mentions. I am glad PEP8 now agrees with me.

[brendanator]

Whether or not SPLIT_BEFORE_BINARY_OPERATOR is implemented, SPLIT_BEFORE_ARITHMETIC_OPERATOR is probably needed

[alok]

@gwelymernans Any chance of implementing split before arithmetic operators? If not, I'd be happy to take a crack at it. Which files contain the necessary logic?

[bwendling]

Hi @alok, I've been swamped at work, so I haven't been able to focus on YAPF in a while. If you'd like to take a crack at it, I would appreciate it.

A lot of the formatting decisions are made in format_decision_state.py. The split_penalty.py file is also a good candidate, as it increases split penalties for different decisions.

[alok]

@gwelymernans I think I'd just model it based off of the split rule for bitwise operators, though I wonder if the fact that both + and - can be used as unary operators complicates things.