Fix zulip unstable formatting with end-of-line comments (#6386)

## Bug

Given
```python
x = () - (#
)
```
the comment is a dangling comment of the empty tuple. This is an
end-of-line comment so it may move after the expression. It still
expands the parent, so the operator breaks:
```python
x = (
    ()
    - ()  #
)
```
In the next formatting pass, the comment is not a trailing tuple but a
trailing bin op comment, so the bin op doesn't break anymore. The
comment again expands the parent, so we still add the superfluous
parentheses
```python
x = (
    () - ()  #
)
```

## Fix

The new formatting is to keep the comment on the empty tuple. This is a
log uglier and again has additional outer parentheses, but it's stable:
```python
x = (
    ()
    - (  #
    )
)
```

## Alternatives

Black formats all the examples above as
```python
x = () - ()  #
```
which i find better. 

I would be happy about any suggestions for better solutions than the
current one. I'd mainly need a workaround for expand parent having an
effect on the bin op instead of first moving the comment to the end and
then applying expand parent to the assign statement.
This commit is contained in:
konsti 2023-08-08 11:15:35 +02:00 committed by GitHub
parent 2bd345358f
commit 90ba40c23c
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11 changed files with 73 additions and 8 deletions

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@ -253,6 +253,16 @@ impl<'ast> Format<PyFormatContext<'ast>> for EmptyWithDanglingComments<'_> {
self.opening,
// end-of-line comments
trailing_comments(&self.comments[..end_of_line_split]),
// Avoid unstable formatting with
// ```python
// x = () - (#
// )
// ```
// Without this the comment would go after the empty tuple first, but still expand
// the bin op. In the second formatting pass they are trailing bin op comments
// so the bin op collapse. Suboptimally we keep parentheses around the bin op in
// either case.
(!self.comments[..end_of_line_split].is_empty()).then_some(hard_line_break()),
// own line comments, which need to be indented
soft_block_indent(&dangling_comments(&self.comments[end_of_line_split..])),
self.closing