Formatter instability: hard line break in binary expression

[charliermarsh]

Given:

[
    1 for components in  # pylint: disable=undefined-loop-variable
    b +  # integer 1 may only have decimal 01-09
    c  # negative decimal
] 

We format as:

[
    1
    for components in b  # pylint: disable=undefined-loop-variable  # integer 1 may only have decimal 01-09
    +
    c  # negative decimal
]

(Notice the hard line break between + and c # negative decimal.)

Which is then formatted as:

[
    1
    for components in b  # pylint: disable=undefined-loop-variable  # integer 1 may only have decimal 01-09
    + c  # negative decimal
]

Sourced from #7590 (A_1798643049678049848.py).

[MichaReiser]

I updated the summary with a smaller reproduction

[charliermarsh]

Thanks.

[charliermarsh]

Didn't mean to assign to myself 😂

[MichaReiser]

I think this is caused by #6588

The binary expression can have dangling comments and it formats them manually. But it's the comprehension that attaches a dangling comment to the binary expression, which messes up the formatting logic, which it intends to handle but the binary expression formatting can't know that.

The solution here is that comprehension shouldn't attach dangling comments to other nodes.

[charliermarsh]

I can fix that.

[MichaReiser]

The problematic comment placement are:

ruff/crates/ruff_python_formatter/src/comments/placement.rs

Lines 2124 to 2131 in 865c898

	if comment.start() < comprehension.iter.start() {
	return if is_own_line {
	CommentPlacement::Default(comment)
	} else {
	// after the `in` but same line, turn into trailing on the `in` token
	CommentPlacement::dangling(&comprehension.iter, comment)
	};
	}

Not related to this issue but has the same underlying problem

ruff/crates/ruff_python_formatter/src/comments/placement.rs

Lines 2158 to 2164 in 865c898

	if is_own_line {
	if last_end < comment.start() && comment.start() < if_token.start() {
	return CommentPlacement::dangling(if_node, comment);
	}
	} else if if_token.start() < comment.start() && comment.start() < if_node.start() {
	return CommentPlacement::dangling(if_node, comment);
	}