Form feeds make the RUF072 fix introduce syntax errors

[dscorbett]

Summary

The useless-finally (RUF072) fix introduces a syntax error when it dedents a try block some but not all of whose lines begin with form feeds.

$ printf 'try:\n\f    1\n    2\nfinally:\n    pass\n' >ruf072.py

$ ruff --isolated check ruf072.py --select RUF072 --preview --fix

error: Fix introduced a syntax error. Reverting all changes.

This indicates a bug in Ruff. If you could open an issue at:

    https://github.com/astral-sh/ruff/issues/new?title=%5BFix%20error%5D

...quoting the contents of `ruf072.py`, the rule codes RUF072, along with the `pyproject.toml` settings and executed command, we'd be very appreciative!

error[RUF072]: Empty `finally` clause
 --> ruf072.py:4:1
  |
2 |       1
3 |       2
4 | / finally:
5 | |     pass
  | |________^
  |
help: Remove the `finally` clause
  - try:
  -     1
1 + 1
2 |     2
  - finally:
  -     pass

Found 1 error.
[*] 1 fixable with the `--fix` option.

Version

ruff 0.15.8 (c2a8815 2026-03-26)

[second-ed]

Hey I think I should be able to take this one, Please could you assign me? I don't seem to have the perms

[second-ed]

The issue is that when it tries to dedent here the line with the formfeed has len 5 whereas the next line has a len of 4 which means that the check on the line I linked passes and the full line is appended.

This change will probably have a large blast radius as a lot of other functions rely on it. A couple of options I thought of (some are quite aggressive):

• add the formfeed to this check here so that this fix is done with libCST
• strip all formfeeds from the start of lines before trying the dedent (potentially just remove all formfeeds - this may mess with the indexing for the locators)
• round the indent values to the nearest multiple of the indent e.g. 5 -> 4 (this seems like a worse option)

Let me know which of these options works best or if theres any others I should consider

[MichaReiser]

Our lexer treats form feeds at the start of a line as if they reset the indentation to root.

ruff/crates/ruff_python_parser/src/lexer.rs

Lines 296 to 300 in 343dad3

	// Form feed
	'\x0C' => {
	self.cursor.bump();
	indentation = Indentation::root();
	}

Which is in line with the spec

A formfeed character may be present at the start of the line; it will be ignored for the indentation calculations above. Formfeed characters occurring elsewhere in the leading whitespace have an undefined effect (for instance, they may reset the space count to zero).

and what CPython does

https://github.com/python/cpython/blob/2cf6a68f028da164bdb9b0ce8ad2cc9bf8f72750/Parser/lexer/lexer.c#L529-L531

So I think it makes sense if dedent just skips them at the start of the line.

[second-ed]

Sweet sounds good, I'll make those changes and push them up - cheers for your help 😃