Python中将字符串转换为全部小写的最有效方法是删除所有非ascii字母字符？

我需要在Python中执行一个简单的任务,即将字符串转换为全部小写并删除所有非ascii非字母字符.

例如:

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

我有一个简单的功能来做到这一点:

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)

但我不禁想到有更高效,更优雅的方式.

谢谢!

编辑:

感谢所有回答的人.我学会了,并且在某些情况下重新学习了很多python.

另一个解决方案(不是pythonic,但非常快)是使用string.translate - 但请注意,这不适用于unicode.值得注意的是,您可以通过将字符移动到一个集合(通过哈希查找,而不是每次执行线性搜索)来加速Dana的代码.以下是我给出的各种解决方案的时间安排:

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))

这给了我:

               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

[编辑]也更新了过滤器解决方案.(注意,使用set.__contains__在这里有很大的不同,因为它避免了为lambda进行额外的函数调用.

>>> filter(str.isalpha, "This is a Test").lower()
'thisisatest'
>>> filter(str.isalpha, "A235th@#$&( er Ra{}|?>ndom").lower()
'atherrandom'

不是特别的运行时效率,但在糟糕,疲惫的编码器眼睛肯定更好:

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

我会:

小写字符串

更换所有[^a-z]有""

像那样:

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

编辑:事实证明原始版本(下面)非常慢,但通过将其转换为闭包(上图)可以获得一些性能.

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

我的性能测量结果是针对字符串的100,000次迭代

"A235th@#$&( er Ra{}|?>ndom"

透露:

f_re_0 took 2672.000 ms (这是这个答案的原始版本)

f_re_1 took 2109.000 ms (这是上面显示的关闭版本)

f_re_2 took 2031.000 ms(关闭版本,没有冗余strip())

f_fl_1 took 1953.000 ms(放松filter/ lambda版本)

f_fl_2 took 1485.000 ms(Coady的filter版本)

f_jn_1 took 1860.000 ms(达娜的join版本)

为了测试,我没有print得到结果.

Python 2.x translate方法

转换为小写并过滤非ascii非字母字符:

from string import ascii_letters, ascii_lowercase, maketrans

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

print "A235th@#$&( er Ra{}|?>ndom".translate(table, deletechars)
# -> 'atherrandom'

Python 3 translate方法

过滤非ascii:

ascii_bytes = "A235th@#$&(???? er Ra{}|?>ndom".encode('ascii', 'ignore')

使用bytes.translate()转换为小写和删除非字母字节:

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'