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

In response to your clarification…

You could use ctypes to execute the c function ‘strcasecmp’. Ctypes is included in Python 2.5. It provides the ability to call out to dll and shared libraries such as libc. Here is a quick example (Python on Linux; see link for Win32 help):

from ctypes import * libc = CDLL('libc.so.6')  // see link above for Win32 help libc.strcasecmp('THIS', 'this') // returns 0 libc.strcasecmp('THIS', 'THAT') // returns 8 

may also want to reference strcasecmp documentation

Not really sure this is any faster or slower (have not tested), but it’s a way to use a C function to do case insensitive string comparisons.

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ActiveState Code – Recipe 194371: Case Insensitive Strings is a recipe for creating a case insensitive string class. It might be a bit over kill for something quick, but could provide you with a common way of handling case insensitive strings if you plan on using them often.