This required some hackery. Long story short, we need to reuse the ability to
omit rightmost bracket pairs (which glues them together and splits on something
else instead), for use with pre-existing trailing commas.
This form of user-controlled formatting is brittle so we have to be careful not
to cause a scenario where Black first formats code without trailing commas in
one way, and then looks at the same file with pre-existing trailing commas
(that it itself put on the previous run) and decides to format the code again.
One particular ugly edge case here is handling of optional parentheses. In
particular, the long-standing `line_length=1` hack got in the way of
pre-existing trailing commas and had to be removed. Instead, a more
intelligent but costly solution was put in place: a "second opinion" if the
formatting that omits optional parentheses ended up causing lines to be too
long. Again, for efficiency purposes, Black reuses Leaf objects from blib2to3
and modifies them in place, which was invalid for having two separate
formattings. Line cloning was used to mitigate this.
Fixes#1619
- when a trailing comma is specified in any bracket pair, that signals to Black
that this bracket pair needs to be always exploded, e.g. presented as "one
item per line";
- this causes some changes to previously formatted code that erroneously left
trailing commas embedded into single-line expressions;
- internally, Black needs to be able to identify trailing commas that it put
itself compared to pre-existing trailing commas. We do this by using/abusing
lib2to3's `was_checked` attribute. It's True for internally generated
trailing commas and False for pre-existing ones (in fact, for all
pre-existing leaves and nodes).
Fixes#1288
Modified maybe_remove_trailing_comma to remove trailing commas for
typedarglists (in addition to arglists), and updated line split logic
to ensure that all lines in a function definition that contain only one
arg have a trailing comma.
