ruff suggests invalid code when fixing UP032

[dima179-fuse]

Summary

Consider the following code:

if __name__ == "__main__":
    number = 0
    string = "{}".format(number := number + 1)
    print(string)

It executes successfully and prints 1.

When I run ruff, it returns

bad_ruff.py:3:14: UP032 [*] Use f-string instead of `format` call
  |
1 | if __name__ == "__main__":
2 |     number = 0
3 |     string = "{}".format(number := number + 1)
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ UP032
4 |     print(string)
  |
  = help: Convert to f-string

When I run ruff check --fix bad_ruff.py, the resulting code is invalid and results in ValueError:

if __name__ == "__main__":
    number = 0
    string = f"{number := number + 1}"
    print(string)

Version

ruff 0.11.13 (5faf72a 2025-06-05)

[dima179-fuse]

Checked and the issue also happens on ruff 0.14.1 (2bffef596 2025-10-16)

[ntBre]

Thanks for the report! Looks like we can't just insert the assignment expression straight into the f-string. Parenthesizing it should be enough to fix the error:

>>> number = 0
>>> f"{number := number + 1}"
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    f"{number := number + 1}"
      ^^^^^^^^^^^^^^^^^^^^^^
ValueError: Invalid format specifier '= number + 1' for object of type 'int'
>>> f"{(number := number + 1)}"
'1'