python - 遍历生成器和转换为列表之间的区别

Question

我本以为这两段代码会产生相同的结果

from itertools import groupby

for i in list(groupby('aaaabb')):
    print i[0], list(i[1])

for i, j in groupby('aaaabb'):
    print i, list(j)

在一个中，我将 groupby 返回的迭代器转换为一个列表并对其进行迭代，在另一个中，我直接迭代返回的迭代器。

这个脚本的输出是

a []
b ['b']


a ['a', 'a', 'a', 'a']
b ['b', 'b']

为什么会这样？

编辑:作为引用，groupby('aabbaa')的结果看起来像

('a', <itertools._grouper object at 0x10c1324d0>)
('b', <itertools._grouper object at 0x10c132250>)

Answer 1

这是 groupby 函数的一个怪癖，大概是为了性能。

来自itertools.groupby documentation :

The returned group is itself an iterator that shares the underlying iterable with groupby(). Because the source is shared, when the groupby() object is advanced, the previous group is no longer visible. So, if that data is needed later, it should be stored as a list:

groups = []
uniquekeys = []
data = sorted(data, key=keyfunc)
for k, g in groupby(data, keyfunc):
    groups.append(list(g))      # Store group iterator as a list
    uniquekeys.append(k)

所以，你可以这样做:

for i in [x, list(y) for x, y in groupby('aabbaa')]:
    print i[0], i[1]