Summary

The fix for mutable-fromkeys-value (RUF024) changes the program’s behavior when the second argument to dict.fromkeys uses a variable named key because the fix creates a new key that shadows it. The best solution is for Ruff to use a gensym-like function to try e.g. key then key_0, key_1, etc. until it finds an unused name. This would benefit all fixes that create new variables. Example:

$ cat >ruf024.py <<'# EOF'
key = "xy"
print(dict.fromkeys("ABC", list(key)))
# EOF

$ python ruf024.py
{'A': ['x', 'y'], 'B': ['x', 'y'], 'C': ['x', 'y']}

$ ruff --isolated check ruf024.py --select RUF024 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ cat ruf024.py
key = "xy"
print({key: list(key) for key in "ABC"})

$ python ruf024.py
{'A': ['A'], 'B': ['B'], 'C': ['C']}

Version

ruff 0.15.8 (c2a8815 2026-03-26)