What is the most efficient way in Python to convert a string to all lowercase stripping out all non-ascii alpha characters?

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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"
"[email protected]#$&( 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!


Edit:

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

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="[email protected]#$&( 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.

>>> filter(str.isalpha, "This is a Test").lower()
'thisisatest'
>>> filter(str.isalpha, "[email protected]#$&( er Ra{}|?>ndom").lower()
'atherrandom'

Not especially runtime efficient, but certainly nicer on poor, tired coder eyes:

def strip_string_and_lowercase(s):
    return ''.join(c for c in s.lower() if c in string.ascii_lowercase)

I would:

  • lowercase the string
  • replace all [^a-z] with ""

Like that:

def strip_string_to_lowercase():
  nonascii = re.compile('[^a-z]')
  return lambda s: nonascii.sub('', s.lower().strip())

EDIT: It turns out that the original version (below) is really slow, though some performance can be gained by converting it into a closure (above).

def strip_string_to_lowercase(s):
  return re.sub('[^a-z]', '', s.lower().strip())

My performance measurements with 100,000 iterations against the string

"[email protected]#$&( er Ra{}|?>ndom"

revealed that:

For the sake of the test, I did not print the results.

Python 2.x translate method

Convert to lowercase and filter non-ascii non-alpha characters:

from string import ascii_letters, ascii_lowercase, maketrans

table = maketrans(ascii_letters, ascii_lowercase*2)
deletechars="".join(set(maketrans('','')) - set(ascii_letters))

print "[email protected]#$&( er Ra{}|?>ndom".translate(table, deletechars)
# -> 'atherrandom'

Python 3 translate method

Filter non-ascii:

ascii_bytes = "[email protected]#$&(???? er Ra{}|?>ndom".encode('ascii', 'ignore')

Use bytes.translate() to convert to lowercase and delete non-alpha bytes:

from string import ascii_letters, ascii_lowercase

alpha, lower = [s.encode('ascii') for s in [ascii_letters, ascii_lowercase]]
table = bytes.maketrans(alpha, lower*2)           # convert to lowercase
deletebytes = bytes(set(range(256)) - set(alpha)) # delete nonalpha

print(ascii_bytes.translate(table, deletebytes))
# -> b'atherrandom'

Similar to @Dana’s, but I think this sounds like a filtering job, and that should be visible in the code. Also without the need to explicitly call join():

def strip_string_to_lowercase(s):
  return filter(lambda x: x in string.ascii_lowercase, s.lower())

I added the filter solutions to Brian’s code:

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="[email protected]#$&( er Ra{}|?>ndom"

def test_original(s):
    tmpStr = s.lower().strip()
    retStrList = []
    for x in tmpStr:
        if x in string.ascii_lowercase:
            retStrList.append(x)

    return ''.join(retStrList)


def test_regex(s):
    return re.sub('[^a-z]', '', s.lower())

def test_regex_closure(s):
  nonascii = re.compile('[^a-z]')
  def replacer(s):
    return nonascii.sub('', s.lower().strip())
  return replacer(s)


def test_str_join(s):
    return ''.join(c for c in s.lower() if c in string.ascii_lowercase)

def test_str_join_set(s):
    return ''.join(c for c in s.lower() if c in letter_set)

def test_filter_set(s):
    return filter(letter_set.__contains__, s.lower())

def test_filter_isalpha(s):
    return filter(str.isalpha, s).lower()

def test_filter_lambda(s):
    return filter(lambda x: x in string.ascii_lowercase, s.lower())

def test_translate(s):
    return string.translate(s, tab, deletions)

for test in sorted(globals()):
    if test.startswith("test_"):
        print "%30s : %s" % (test, timeit.Timer("f(s)", 
              "from __main__ import %s as f, s" % test).timeit(200000))

This gives me:

       test_filter_isalpha : 1.31981746283
        test_filter_lambda : 2.23935583992
           test_filter_set : 0.76511679557
             test_original : 2.13079176264
                test_regex : 2.44295629752
        test_regex_closure : 2.65205913042
             test_str_join : 2.25571266739
         test_str_join_set : 1.75565888961
            test_translate : 0.269259640541

It appears that isalpha is using a similar algorithm, at least in terms of O(), to the set algorithm.


Edit:
Added the filter set, and renamed the filter functions to be a little more clear.

This is a typical application of list compehension:

import string
s = "[email protected]#$&( er Ra{}|?<ndom"
print ''.join(c for c in s.lower() if c in string.ascii_lowercase)

It won’t filter out “<” (html entity), as in your example, but I assume that was accidental cut and past problem.

>>> import string
>>> a = "[email protected]#$&( er Ra{}|?&lt;ndom"
>>> ''.join(i for i in a.lower() if i in string.ascii_lowercase)
'otheraltndom'

doing essentially the same as you.

Python 2.x:

import string
valid_chars= string.ascii_lowercase + string.ascii_uppercase

def only_lower_ascii_alpha(text):
    return filter(valid_chars.__contains__, text).lower()

Works with either str or unicode arguments.

>>> only_lower_ascii_alpha("Hello there 123456!")
'hellothere'
>>> only_lower_ascii_alpha(u"435 café")
u'caf'

Personally I would use a regular expression and then convert the final string to lower case. I have no idea how to write it in Python, but the basic idea is to:

  1. Remove characters in string that don’t match case-insensitive regex “\w
  2. Convert string to lower-case

or vice versa.


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