我有这样的代码:

good = [x for x in mylist if x in goodvals]
bad = [x for x in mylist if x not in goodvals]

目标是根据mylist的内容是否满足条件,将它们拆分为另外两个列表。

我怎样才能做得更优雅呢?我能避免在mylist上做两个单独的迭代吗?我可以通过这样做来提高性能吗?


当前回答

还有另一个答案,简短但“邪恶”(用于理解列表的副作用)。

digits = list(range(10))
odd = [x.pop(i) for i, x in enumerate(digits) if x % 2]

>>> odd
[1, 3, 5, 7, 9]

>>> digits
[0, 2, 4, 6, 8]

其他回答

def partition(pred, seq):
  return reduce( lambda (yes, no), x: (yes+[x], no) if pred(x) else (yes, no+[x]), seq, ([], []) )

所有提出的解决方案的问题是,它将扫描和应用过滤功能两次。我会做一个简单的小函数,像这样:

def split_into_two_lists(lst, f):
    a = []
    b = []
    for elem in lst:
        if f(elem):
            a.append(elem)
        else:
            b.append(elem)
    return a, b

这样你就不会重复处理任何东西,也不会重复代码。

有时候,列表理解并不是最好的选择!

我根据人们对这个话题的回答做了一个小测试,在一个随机生成的列表上测试。以下是列表的生成(可能有更好的方法,但这不是重点):

good_list = ('.jpg','.jpeg','.gif','.bmp','.png')

import random
import string
my_origin_list = []
for i in xrange(10000):
    fname = ''.join(random.choice(string.lowercase) for i in range(random.randrange(10)))
    if random.getrandbits(1):
        fext = random.choice(good_list)
    else:
        fext = "." + ''.join(random.choice(string.lowercase) for i in range(3))

    my_origin_list.append((fname + fext, random.randrange(1000), fext))

好了

# Parand
def f1():
    return [e for e in my_origin_list if e[2] in good_list], [e for e in my_origin_list if not e[2] in good_list]

# dbr
def f2():
    a, b = list(), list()
    for e in my_origin_list:
        if e[2] in good_list:
            a.append(e)
        else:
            b.append(e)
    return a, b

# John La Rooy
def f3():
    a, b = list(), list()
    for e in my_origin_list:
        (b, a)[e[2] in good_list].append(e)
    return a, b

# Ants Aasma
def f4():
    l1, l2 = tee((e[2] in good_list, e) for e in my_origin_list)
    return [i for p, i in l1 if p], [i for p, i in l2 if not p]

# My personal way to do
def f5():
    a, b = zip(*[(e, None) if e[2] in good_list else (None, e) for e in my_origin_list])
    return list(filter(None, a)), list(filter(None, b))

# BJ Homer
def f6():
    return filter(lambda e: e[2] in good_list, my_origin_list), filter(lambda e: not e[2] in good_list, my_origin_list)

使用cmpthese函数,最好的结果是dbr答案:

f1     204/s  --    -5%   -14%   -15%   -20%   -26%
f6     215/s     6%  --    -9%   -11%   -16%   -22%
f3     237/s    16%    10%  --    -2%    -7%   -14%
f4     240/s    18%    12%     2%  --    -6%   -13%
f5     255/s    25%    18%     8%     6%  --    -8%
f2     277/s    36%    29%    17%    15%     9%  --

还有另一个答案,简短但“邪恶”(用于理解列表的副作用)。

digits = list(range(10))
odd = [x.pop(i) for i, x in enumerate(digits) if x % 2]

>>> odd
[1, 3, 5, 7, 9]

>>> digits
[0, 2, 4, 6, 8]

受到@gnibbler伟大(但简洁!)回答的启发,我们可以应用该方法映射到多个分区:

from collections import defaultdict

def splitter(l, mapper):
    """Split an iterable into multiple partitions generated by a callable mapper."""

    results = defaultdict(list)

    for x in l:
        results[mapper(x)] += [x]

    return results

然后可以使用splitter,如下所示:

>>> l = [1, 2, 3, 4, 2, 3, 4, 5, 6, 4, 3, 2, 3]
>>> split = splitter(l, lambda x: x % 2 == 0)  # partition l into odds and evens
>>> split.items()
>>> [(False, [1, 3, 3, 5, 3, 3]), (True, [2, 4, 2, 4, 6, 4, 2])]

这适用于有更复杂映射的两个以上分区(也适用于迭代器):

>>> import math
>>> l = xrange(1, 23)
>>> split = splitter(l, lambda x: int(math.log10(x) * 5))
>>> split.items()
[(0, [1]),
 (1, [2]),
 (2, [3]),
 (3, [4, 5, 6]),
 (4, [7, 8, 9]),
 (5, [10, 11, 12, 13, 14, 15]),
 (6, [16, 17, 18, 19, 20, 21, 22])]

或者用字典来映射:

>>> map = {'A': 1, 'X': 2, 'B': 3, 'Y': 1, 'C': 2, 'Z': 3}
>>> l = ['A', 'B', 'C', 'C', 'X', 'Y', 'Z', 'A', 'Z']
>>> split = splitter(l, map.get)
>>> split.items()
(1, ['A', 'Y', 'A']), (2, ['C', 'C', 'X']), (3, ['B', 'Z', 'Z'])]