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
解决方案
from itertools import tee
def unpack_args(fn):
return lambda t: fn(*t)
def separate(fn, lx):
return map(
unpack_args(
lambda i, ly: filter(
lambda el: bool(i) == fn(el),
ly)),
enumerate(tee(lx, 2)))
test
[even, odd] = separate(
lambda x: bool(x % 2),
[1, 2, 3, 4, 5])
print(list(even) == [2, 4])
print(list(odd) == [1, 3, 5])
其他回答
Good = [x for x in mylist if x in goodvals] Bad = [x for x in mylist if x not in goodvals] 我怎样才能做得更优雅呢?
代码已经非常优雅了。
使用集合可能会有轻微的性能改进,但差异是微不足道的。基于集合的方法也会丢弃重复项,并且不会保留元素的顺序。我发现列表理解也更容易阅读。
事实上,我们甚至可以更简单地使用for循环:
good, bad = [], []
for x in mylist:
if x in goodvals:
good.append(f)
else:
bad.append(f)
这种方法可以更容易地添加额外的逻辑。例如,代码很容易被修改为丢弃None值:
good, bad = [], []
for x in mylist:
if x is None:
continue
if x in goodvals:
good.append(f)
else:
bad.append(f)
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)
如果你不介意使用一个外部库,有两个我知道本机实现这个操作:
>>> files = [ ('file1.jpg', 33, '.jpg'), ('file2.avi', 999, '.avi')]
>>> IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
iteration_utilities.partition: >>> from iteration_utilities import partition >>> notimages, images = partition(files, lambda x: x[2].lower() in IMAGE_TYPES) >>> notimages [('file2.avi', 999, '.avi')] >>> images [('file1.jpg', 33, '.jpg')] more_itertools.partition >>> from more_itertools import partition >>> notimages, images = partition(lambda x: x[2].lower() in IMAGE_TYPES, files) >>> list(notimages) # returns a generator so you need to explicitly convert to list. [('file2.avi', 999, '.avi')] >>> list(images) [('file1.jpg', 33, '.jpg')]
清晰快速
这个列表理解是简单的阅读和快速。这正是上级要求的。
set_good_vals = set(good_vals) # Speed boost.
good = [x for x in my_list if x in set_good_vals]
bad = [x for x in my_list if x not in set_good_vals]
我更喜欢一个列表理解而不是两个,但不像张贴的许多答案(其中一些相当巧妙),它是可读的和清晰的。这也是网页上最快的答案之一。
唯一(稍微)快一点的答案是:
set_good_vals = set(good_vals)
good, bad = [], []
for item in my_list:
_ = good.append(item) if item in set_good_vals else bad.append(item)
...还有它的变体。(见我的另一个答案)。但我觉得第一种方法更优雅,而且几乎一样快。
这个问题已经有很多答案了,但似乎都不如我最喜欢的解决这个问题的方法,这种方法只遍历和测试每个项目一次,并使用列表理解的速度来构建两个输出列表之一,因此它只需要使用相对较慢的附加来构建一个输出列表:
bad = []
good = [x for x in mylist if x in goodvals or bad.append(x)]
In my answer to a similar question, I explain how this approach works (a combination of Python's greedy evaluation of or refraining from executing the append for "good" items, and append returning a false-like value which leaves the if condition false for "bad" items), and I show timeit results indicating that this approach outcompetes alternatives like those suggested here, especially in cases where the majority of items will go into the list built by list-comprehension (in this case, the good list).