[ruff] Needless else clause (RUF047)

[InSyncWithFoo]

Summary

Partially addresses #13929.

Test Plan

cargo nextest run and cargo insta test.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+6 -0 violations, +0 -0 fixes in 2 projects; 53 projects unchanged)

apache/airflow (+4 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview --select ALL

+ providers/src/airflow/providers/google/cloud/operators/bigquery.py:1638:9: RUF047 [*] Empty `else` clause
+ providers/tests/fab/auth_manager/api_endpoints/test_asset_endpoint.py:38:5: RUF047 [*] Empty `else` clause
+ providers/tests/fab/auth_manager/api_endpoints/test_dag_run_endpoint.py:44:5: RUF047 [*] Empty `else` clause
+ tests/models/test_taskinstance.py:2388:17: RUF047 [*] Empty `else` clause

lnbits/lnbits (+2 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview

+ lnbits/core/crud.py:791:5: RUF047 [*] Empty `else` clause
+ lnbits/core/crud.py:800:5: RUF047 [*] Empty `else` clause

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
RUF047	6	6	0	0	0

[MichaReiser]

Thanks.

I refactored your rule to remove some duplication and simplified it to only detect single pass or ... statements. PIE790 detects bodies containing both a ... and a pass statement.

I further extended the test cases and there's one case where we fail to detect a useless else because of the comment and one case where we marked the else as useless when we should not. We have to take the indent of the comments into consideration. See

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

Line 552 in 9f3a38d

fn handle_own_line_comment_between_branches<'a>(

and

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

Line 641 in 9f3a38d

fn handle_own_line_comment_after_branch<'a>(

Maybe we can come up with a simpler heuristic for this specific case?

[InSyncWithFoo]

@MichaReiser If indentation is to be taken into consideration, should this be reported or not?

if test:
    _4_spaces()
  # 2 spaces
else:
    ...

What about this?

if test:
	_1_tab()  # These two lines would align if tabs are displayed as 1 character wide.
 # 1 space
else:
	...

# Same question, but for 2, 3, 4 spaces?

[MichaReiser]

We should implement it so that it's consistent with the formatter. Your examples get formatted to

if test:
    _1_tab()  # These two lines would align if tabs are displayed as 1 character wide.
    # 1 space
else:
    ...

# Same question, but for 2, 3, 4 spaces?

if test:
    _4_spaces()
# 2 spaces
else:
    ...

[MichaReiser]

@InSyncWithFoo do you plan to follow up on this PR?

[InSyncWithFoo]

@MichaReiser Yes.

[InSyncWithFoo]

Took me long enough. I think I got most of it, save for this one:

if a:
	b()
else:
	...
	# comment

I have no idea how to detect that comment: it's not included within else_range. I thought about manual tokens processing, but then I wouldn't know when to stop, because there might not be a next statement at all. Iterating all comments is a lot more expensive than necessary, so that's not a good choice either.

[MichaReiser]

I have no idea how to detect that comment: it's not included within else_range. I thought about manual tokens processing, but then I wouldn't know when to stop, because there might not be a next statement at all. Iterating all comments is a lot more expensive than necessary, so that's not a good choice either.

I would use the tokens and start at the end of the else_range and skip all tokens until it finds the first new line token. From there, keep taking all tokens until you find the first non-trivia token that isn't a comment. For all comments in that range, test if the indent of the comment is larger than the indent of the else header. If so -> it belongs to the else body. You can stop as soon as you've found the first comment that doesn't belong to the else body.

[MichaReiser]

Nice