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
这里已经有很多解了,但另一种方法是
anims = []
images = [f for f in files if (lambda t: True if f[2].lower() in IMAGE_TYPES else anims.append(t) and False)(f)]
只在列表上迭代一次,看起来更python化,因此对我来说是可读的。
>>> files = [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ('file1.bmp', 33L, '.bmp')]
>>> IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
>>> anims = []
>>> images = [f for f in files if (lambda t: True if f[2].lower() in IMAGE_TYPES else anims.append(t) and False)(f)]
>>> print '\n'.join([str(anims), str(images)])
[('file2.avi', 999L, '.avi')]
[('file1.jpg', 33L, '.jpg'), ('file1.bmp', 33L, '.bmp')]
>>>
其他回答
如果列表由组和间歇分隔符组成,您可以使用:
def split(items, p):
groups = [[]]
for i in items:
if p(i):
groups.append([])
groups[-1].append(i)
return groups
用法:
split(range(1,11), lambda x: x % 3 == 0)
# gives [[1, 2], [3, 4, 5], [6, 7, 8], [9, 10]]
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。
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)
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).
这里已经有很多解了,但另一种方法是
anims = []
images = [f for f in files if (lambda t: True if f[2].lower() in IMAGE_TYPES else anims.append(t) and False)(f)]
只在列表上迭代一次,看起来更python化,因此对我来说是可读的。
>>> files = [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ('file1.bmp', 33L, '.bmp')]
>>> IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
>>> anims = []
>>> images = [f for f in files if (lambda t: True if f[2].lower() in IMAGE_TYPES else anims.append(t) and False)(f)]
>>> print '\n'.join([str(anims), str(images)])
[('file2.avi', 999L, '.avi')]
[('file1.jpg', 33L, '.jpg'), ('file1.bmp', 33L, '.bmp')]
>>>