The first sentence sounds as if they modified the implementemention of str.lower(). That would be bonkers, but that's not what they did. https://github.com/pyt

The first sentence sounds as if they modified the implementemention of str.lower(). That would be bonkers, but that's not what they did. https://github.com/python/cpython/commit/7e109d084d55e7eb https://github.com/python/cpython/commit/7e109d084d55e7eb The important part is: # B.3 is mostly Python's .lower, except for a number # of special cases, e.g. considering canonical forms. +# To enforce Unicode 3.2.0 behavior of .lower instead of +# whatever Unicode version is included with Python we +# add unassigned or newly case-folding codepoints to +# the exception map, too. b3_exceptions = {} for k,v in table_b2.items(): if list(map(ord, chr(k).lower())) != v: b3_exceptions[k] = "".join(map(chr,v)) +for cp in range(0x110000): + ch = chr(cp) + # Assigned in current Unicode version + # and supports case folding, but not + # explicitly in B.2 or B.3 tables. + if (unicodedata_current.category(ch) != "Cn" + and ch.lower() != ch + and cp not in table_b2 + and cp not in table_b3): + b3_exceptions[cp] = ch # Identity.