Question

I have a simple task I need to perform in Python, which is to convert a string to all lowercase and strip out all non-ascii non-alpha characters.

For example:

'This is a Test' -> 'thisisatest' 'A235th@#$&( er Ra{}|?>ndom' -> 'atherrandom' 

I have a simple function to do this:

import string import sys  def strip_string_to_lowercase(s):     tmpStr = s.lower().strip()     retStrList = []     for x in tmpStr:         if x in string.ascii_lowercase:             retStrList.append(x)      return ''.join(retStrList) 

But I cannot help thinking there is a more efficient, or more elegant, way.

Thanks!

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Edit:

Thanks to all those that answered. I learned, and in some cases re-learned, a good deal of python.

Answer 1

Another solution (not that pythonic, but very fast) is to use string.translate – though note that this will not work for unicode. It’s also worth noting that you can speed up Dana’s code by moving the characters into a set (which looks up by hash, rather than performing a linear search each time). Here are the timings I get for various of the solutions given:

import string, re, timeit  # Precomputed values (for str_join_set and translate)  letter_set = frozenset(string.ascii_lowercase + string.ascii_uppercase) tab = string.maketrans(string.ascii_lowercase + string.ascii_uppercase,                        string.ascii_lowercase * 2) deletions = ''.join(ch for ch in map(chr,range(256)) if ch not in letter_set)  s='A235th@#$&( er Ra{}|?>ndom'  # From unwind's filter approach def test_filter(s):     return filter(lambda x: x in string.ascii_lowercase, s.lower())  # using set instead (and contains) def test_filter_set(s):     return filter(letter_set.__contains__, s).lower()  # Tomalak's solution def test_regex(s):     return re.sub('[^a-z]', '', s.lower())  # Dana's def test_str_join(s):     return ''.join(c for c in s.lower() if c in string.ascii_lowercase)  # Modified to use a set. def test_str_join_set(s):     return ''.join(c for c in s.lower() if c in letter_set)  # Translate approach. def test_translate(s):     return string.translate(s, tab, deletions)   for test in sorted(globals()):     if test.startswith('test_'):         assert globals()[test](s)=='atherrandom'         print '%30s : %s' % (test, timeit.Timer('f(s)',                'from __main__ import %s as f, s' % test).timeit(200000)) 

This gives me:

               test_filter : 2.57138351271            test_filter_set : 0.981806765698                 test_regex : 3.10069885233              test_str_join : 2.87172979743          test_str_join_set : 2.43197956381             test_translate : 0.335367566218 

[Edit] Updated with filter solutions as well. (Note that using set.__contains__ makes a big difference here, as it avoids making an extra function call for the lambda.