FURB188 fix should be marked unsafe except with known strs #20724
Description
Summary
The fix for slice-to-remove-prefix-or-suffix (FURB188) should be marked unsafe unless it is statically known that the full string and its affix are both strs. If it is statically known that they are instances of unrelated classes, the rule should be suppressed. Otherwise, the fix can change the program’s behavior. They have to be exactly str for the fix to be safe, not subclasses of str. Example:
$ cat >furb188.py <<'# EOF'
class Seq:
def __init__(self, inner):
self.inner = inner
def startswith(self, prefix):
return tuple(self.inner[:len(prefix)]) == tuple(prefix)
def __getitem__(self, item):
return type(self)(self.inner[item])
seq = Seq(("1", "2", "3", "4", "5"))
try:
if seq.startswith("123"):
seq = seq[3:]
print(seq.inner)
except AttributeError as e:
print(e)
text = "12345"
prefix = ("123",)
try:
if text.startswith(prefix):
text = text[len(prefix):]
print(text)
except TypeError as e:
print(e)
# EOF
$ python furb188.py
('4', '5')
2345
$ ruff --isolated check furb188.py --select FURB188 --fix
Found 2 errors (2 fixed, 0 remaining).
$ python furb188.py
'Seq' object has no attribute 'removeprefix'
removeprefix() argument must be str, not tupleVersion
ruff 0.13.3 (188c0dc 2025-10-02)