How can I do case insensitive string comparison in Python?
I would like to encapsulate comparison of a regular strings to a repository string using in a very simple and Pythonic way. I also would like to have ability to look up values in a dict hashed by strings using regular python strings.
Assuming ASCII strings:
string1 = 'Hello' string2 = 'hello' if string1.lower() == string2.lower(): print("The strings are the same (case insensitive)") else: print("The strings are NOT the same (case insensitive)")
Comparing strings in a case insensitive way seems trivial, but it's not. I will be using Python 3, since Python 2 is underdeveloped here.
The first thing to note is that case-removing conversions in Unicode aren't trivial. There is text for which
text.lower() != text.upper().lower(), such as
"ß".lower() #>>> 'ß' "ß".upper().lower() #>>> 'ss'
But let's say you wanted to caselessly compare
"Buße". Heck, you probably also want to compare
"BUẞE" equal - that's the newer capital form. The recommended way is to use
Return a casefolded copy of the string. Casefolded strings may be used for caseless matching.
Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. [...]
Do not just use
casefold is not available, doing
.upper().lower() helps (but only somewhat).
Then you should consider accents. If your font renderer is good, you probably think
"ê" == "ê" - but it doesn't:
"ê" == "ê" #>>> False
This is because the accent on the latter is a combining character.
import unicodedata [unicodedata.name(char) for char in "ê"] #>>> ['LATIN SMALL LETTER E WITH CIRCUMFLEX'] [unicodedata.name(char) for char in "ê"] #>>> ['LATIN SMALL LETTER E', 'COMBINING CIRCUMFLEX ACCENT']
unicodedata.normalize("NFKD", "ê") == unicodedata.normalize("NFKD", "ê") #>>> True
To finish up, here this is expressed in functions:
import unicodedata def normalize_caseless(text): return unicodedata.normalize("NFKD", text.casefold()) def caseless_equal(left, right): return normalize_caseless(left) == normalize_caseless(right)