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
简单的生成器版本,在内存中保存尽可能少的值,并且只调用pred一次:
from collections import deque
from typing import Callable, TypeVar, Iterable
_T = TypeVar('_T')
def iter_split(pred: Callable[[_T], bool],
iterable: Iterable[_T]) -> tuple[Iterable[_T], Iterable[_T]]:
"""Split an iterable into two iterables based on a predicate.
The predicate will only be called once per element.
Returns:
A tuple of two iterables, the first containing all elements for which
the predicate returned True, the second containing all elements for
which the predicate returned False.
"""
iterator = iter(iterable)
true_values: deque[_T] = deque()
false_values: deque[_T] = deque()
def true_generator():
while True:
while true_values:
yield true_values.popleft()
for item in iterator:
if pred(item):
yield item
break
false_values.append(item)
else:
break
def false_generator():
while True:
while false_values:
yield false_values.popleft()
for item in iterator:
if not pred(item):
yield item
break
true_values.append(item)
else:
break
return true_generator(), false_generator()
其他回答
就我个人而言,我喜欢你引用的版本,假设你已经有了一个好的列表。如果没有,就像这样:
good = filter(lambda x: is_good(x), mylist)
bad = filter(lambda x: not is_good(x), mylist)
当然,这真的非常类似于使用列表理解,就像你最初做的,但用一个函数而不是一个查找:
good = [x for x in mylist if is_good(x)]
bad = [x for x in mylist if not is_good(x)]
总的来说,我发现列表推导式的美学非常令人满意。当然,如果您实际上不需要保留顺序,也不需要重复,那么在集合上使用交集和差分方法也会很好。
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]
例如,按偶数和奇数拆分列表
arr = range(20)
even, odd = reduce(lambda res, next: res[next % 2].append(next) or res, arr, ([], []))
或者概括地说:
def split(predicate, iterable):
return reduce(lambda res, e: res[predicate(e)].append(e) or res, iterable, ([], []))
优点:
最短路径 Predicate对每个元素只应用一次
缺点
需要函数式编程范例的知识
good.append(x) if x in goodvals else bad.append(x)
来自@dansalmo的这个优雅简洁的回答被埋没在评论中,所以我只是把它作为一个答案转发到这里,这样它就能得到应有的重视,尤其是对新读者来说。
完整的例子:
good, bad = [], []
for x in my_list:
good.append(x) if x in goodvals else bad.append(x)
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% --