When the fix for string-dot-format-extra-positional-arguments (F523) removes a string’s format method call, it can change the program’s behavior. If the string contains a double brace like {{, it originally represents an escape sequence for a single brace like {, but with the method call removed it literally represents a double brace. If the string contains an unset field name, removing the method call suppresses the expected KeyError.

$ ruff --version
ruff 0.9.2

$ cat f523.py
print("{{".format("!"))
print("{x}".format("!"))

$ python f523.py
{
Traceback (most recent call last):
  File "f523.py", line 2, in <module>
    print("{x}".format("!"))
          ^^^^^^^^^^^^^^^^^
KeyError: 'x'

$ ruff --isolated check --fix --select F523 f523.py
Found 2 errors (2 fixed, 0 remaining).

$ python f523.py
{{
{x}