Answer by Nathan Craike for How do I do a case-insensitive string comparison?

Using Python 2, calling .lower() on each string or Unicode object...

string1.lower() == string2.lower()

...will work most of the time, but indeed doesn't work in the situations @tchrist has described.

Assume we have a file called unicode.txt containing the two strings Σίσυφος and ΣΊΣΥΦΟΣ. With Python 2:

>>> utf8_bytes = open("unicode.txt", 'r').read()>>> print repr(utf8_bytes)'\xce\xa3\xce\xaf\xcf\x83\xcf\x85\xcf\x86\xce\xbf\xcf\x82\n\xce\xa3\xce\x8a\xce\xa3\xce\xa5\xce\xa6\xce\x9f\xce\xa3\n'>>> u = utf8_bytes.decode('utf8')>>> print uΣίσυφοςΣΊΣΥΦΟΣ>>> first, second = u.splitlines()>>> print first.lower()σίσυφος>>> print second.lower()σίσυφοσ>>> first.lower() == second.lower()False>>> first.upper() == second.upper()True

The Σ character has two lowercase forms, ς and σ, and .lower() won't help compare them case-insensitively.

However, as of Python 3, all three forms will resolve to ς, and calling lower() on both strings will work correctly:

>>> s = open('unicode.txt', encoding='utf8').read()>>> print(s)ΣίσυφοςΣΊΣΥΦΟΣ>>> first, second = s.splitlines()>>> print(first.lower())σίσυφος>>> print(second.lower())σίσυφος>>> first.lower() == second.lower()True>>> first.upper() == second.upper()True

So if you care about edge-cases like the three sigmas in Greek, use Python 3.

(For reference, Python 2.7.3 and Python 3.3.0b1 are shown in the interpreter printouts above.)