On some more digging through history, it looks like the change in behavior for byte strings was intentional: https://github.com/python/cpython/commit/6ccd3f2dbc

On some more digging through history, it looks like the change in behavior for byte strings was intentional: https://github.com/python/cpython/commit/6ccd3f2dbcb98b33a71ffa6eae949deae797c09c https://github.com/python/cpython/commit/6ccd3f2dbcb98b33a71... Author: Guido van Rossum <guido@python.org> Date: Tue Oct 9 03:46:30 2007 +0000 Replace all (locale-dependent) uses of isupper(), tolower(), etc., by locally-defined macros that assume ASCII and only consider ASCII letters.