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?
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
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'])]
其他回答
itertools。Groupby几乎可以满足您的要求,除了它要求对条目进行排序以确保您获得一个连续的范围之外,因此您需要首先根据键进行排序(否则您将为每种类型获得多个交错的组)。如。
def is_good(f):
return f[2].lower() in IMAGE_TYPES
files = [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ('file3.gif', 123L, '.gif')]
for key, group in itertools.groupby(sorted(files, key=is_good), key=is_good):
print key, list(group)
给:
False [('file2.avi', 999L, '.avi')]
True [('file1.jpg', 33L, '.jpg'), ('file3.gif', 123L, '.gif')]
与其他解决方案类似,可以将键func定义为任意数量的组。
如果你坚持聪明,你可以采用温登的解决方案,再加上一点虚假的聪明:
def splay(l, f, d=None):
d = d or {}
for x in l: d.setdefault(f(x), []).append(x)
return d
第一步(pre-OP-edit):使用集合:
mylist = [1,2,3,4,5,6,7]
goodvals = [1,3,7,8,9]
myset = set(mylist)
goodset = set(goodvals)
print list(myset.intersection(goodset)) # [1, 3, 7]
print list(myset.difference(goodset)) # [2, 4, 5, 6]
这对可读性(IMHO)和性能都有好处。
第二步(post-OP-edit):
创建一个好的扩展列表:
IMAGE_TYPES = set(['.jpg','.jpeg','.gif','.bmp','.png'])
这将提高性能。否则,你现在的情况在我看来还不错。
如果你不想用两行代码来完成一个语义只需要一次的操作,你可以把上面的一些方法(甚至是你自己的方法)包装在一个函数中:
def part_with_predicate(l, pred):
return [i for i in l if pred(i)], [i for i in l if not pred(i)]
这不是一种惰性计算方法,它确实对列表进行了两次迭代,但是它允许您在一行代码中对列表进行分区。
一个基于生成器的版本,如果你能忍受一个或两个原始列表的反转。
设置…
random.seed(1234)
a = list(range(10))
random.shuffle(a)
a
[2, 8, 3, 5, 6, 4, 9, 0, 1, 7]
至于分裂……
(list((a.pop(j) for j, y in [(len(a)-i-1, x) for i,x in enumerate(a[::-1])] if y%2 == 0))[::-1], a)
([2, 8, 6, 4, 0], [3, 5, 9, 1, 7])
Another list of tuples of locations and each element is built in reverse order. In a generator wrapped round that each element is tested against the predicate (here test for even) and if True then the element is poped using previously computed locations. We are working backwards along the list so poping elements out does not change positions closer to the beginning of the list. A wrapping list() evaluates the generator and a final revers [::-1] puts the elements back in the right order. The original list "a" now only contains elements that for which the predicate is False.