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This is not a real-world example, but it brokes some invariant (part of path must not contain separator):
>>> pathlib.PurePath('/a/b.c.d').with_suffix('///')
PurePosixPath('/a/b.c///')
>>> pathlib.PurePath('/a/b.c.d').with_suffix('/not/split/into/parts').parts
('/', 'a', 'b.c/not/split/into/parts')
I think these cases should raise an error.
I would also like to consider the following to be an error, since the argument of with_suffix() is not exactly a suffix:
>>> PurePath('/a/b.c.d').with_suffix('e')
PurePosixPath('/a/b.ce')
but I'm far less sure in this case.