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

不确定这是否是一个好方法,但也可以这样做

IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
files = [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi')]
images, anims = reduce(lambda (i, a), f: (i + [f], a) if f[2] in IMAGE_TYPES else (i, a + [f]), files, ([], []))

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。

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()

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。

不确定这是否是一个好方法,但也可以这样做

IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
files = [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi')]
images, anims = reduce(lambda (i, a), f: (i + [f], a) if f[2] in IMAGE_TYPES else (i, a + [f]), files, ([], []))

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]]