W504: yapf inserts line break after rather than before a binary operator

[malcolmr]

I see plenty of issues pointing out that yapf will line-break before binary operators (counter to the formatting originally required by pep8, triggering the off-by-default W503 warning), but I'm seeing the opposite in at least some cases, where the new recommendation isn't being followed, and yapf is still breaking after the operator.

With yapf 0.25.0:

$ cat > /tmp/x.py
def _():
    raise ValueError('This is a long message that ends with an argument: ' + str(42))
$ python -m yapf /tmp/x.py
def _():
    raise ValueError('This is a long message that ends with an argument: ' +
                     str(42))
$

This results in:

$ .python -m pycodestyle --ignore=W503 /tmp/x.py 
/tmp/x.py:2:22: W504 line break after binary operator

[themr0c]

same with flake8==3.6.0