Note about _Py_wchar2char(): it is possible to convert character by character (instead of working on substrings) because the input string doesn't contain surrogate pairs. _Py_char2wchar() ensures the the output string doens't contain surrogate pairs: if a byte sequence produces a surrogate pairs, the byte sequence is encoded using the surrogateescape error handler (U+DC00..U+DCFF range). I should add this note in _Py_wchar2char() comment.