dirs._addpath: don't mutate Python strings after exposing them (issue4589) One of the rules of Python strings is that they're immutable. dirs._addpath breaks this assumption for performance, which is fine as long as it is done safely -- once a string is no longer internal-only it shouldn't be mutated. Unfortunately, we weren't being safe here -- we were mutating 'key' even after adding it to a dictionary. This only really affects other C code that reads strings, so it's somewhat hard to write a test for this without poking into the internal representation of the string via ctypes or similar. There is currently no C code that reads the output of the string, but there will likely be some soon as the bug indicates. There's no significant difference in performance.