Ignore case in Python strings

Question

What is the easiest way to compare strings in Python, ignoring case?

Of course one can do (str1.lower() <= str2.lower()), etc., but this created two additional temporary strings (with the obvious alloc/g-c overheads).

I guess I'm looking for an equivalent to C's stricmp().

[Some more context requested, so I'll demonstrate with a trivial example:]

Suppose you want to sort a looong list of strings. You simply do theList.sort(). This is O(n * log(n)) string comparisons and no memory management (since all strings and list elements are some sort of smart pointers). You are happy.

Now, you want to do the same, but ignore the case (let's simplify and say all strings are ascii, so locale issues can be ignored). You can do theList.sort(key=lambda s: s.lower()), but then you cause two new allocations per comparison, plus burden the garbage-collector with the duplicated (lowered) strings. Each such memory-management noise is orders-of-magnitude slower than simple string comparison.

Now, with an in-place stricmp()-like function, you do: theList.sort(cmp=stricmp) and it is as fast and as memory-friendly as theList.sort(). You are happy again.

The problem is any Python-based case-insensitive comparison involves implicit string duplications, so I was expecting to find a C-based comparisons (maybe in module string).

Could not find anything like that, hence the question here. (Hope this clarifies the question).

Answer 1

Here is a benchmark showing that using str.lower is faster than the accepted answer's proposed method (libc.strcasecmp):

#!/usr/bin/env python2.7
import random
import timeit

from ctypes import *
libc = CDLL('libc.dylib') # change to 'libc.so.6' on linux

with open('/usr/share/dict/words', 'r') as wordlist:
    words = wordlist.read().splitlines()
random.shuffle(words)
print '%i words in list' % len(words)

setup = 'from __main__ import words, libc; gc.enable()'
stmts = [
    ('simple sort', 'sorted(words)'),
    ('sort with key=str.lower', 'sorted(words, key=str.lower)'),
    ('sort with cmp=libc.strcasecmp', 'sorted(words, cmp=libc.strcasecmp)'),
]

for (comment, stmt) in stmts:
    t = timeit.Timer(stmt=stmt, setup=setup)
    print '%s: %.2f msec/pass' % (comment, (1000*t.timeit(10)/10))

typical times on my machine:

235886 words in list
simple sort: 483.59 msec/pass
sort with key=str.lower: 1064.70 msec/pass
sort with cmp=libc.strcasecmp: 5487.86 msec/pass

So, the version with str.lower is not only the fastest by far, but also the most portable and pythonic of all the proposed solutions here. I have not profiled memory usage, but the original poster has still not given a compelling reason to worry about it. Also, who says that a call into the libc module doesn't duplicate any strings?

NB: The lower() string method also has the advantage of being locale-dependent. Something you will probably not be getting right when writing your own "optimised" solution. Even so, due to bugs and missing features in Python, this kind of comparison may give you wrong results in a unicode context.