Python collections.Counter似乎打破了原来的清单 [英] Python collections.Counter seems to break the original list

问题描述

代码:

sortedgroups = sorted(metagroups, key=lambda group: group[-1])
categories = map(operator.itemgetter(-1), sortedgroups)
categorycounts = collections.Counter(categories)

print('Writing output files')

with open('report.txt', 'a+') as f:
    for category in categories:
        f.write(category + '\n')

因此，如果我注释掉，此代码将起作用:

So this code works if I comment out:

categorycounts = collections.Counter(categories)

如果我尝试计算类别列表中相同字符串的数量，则for循环似乎中断. collections.Counter()是否会修改原始类别对象?

The for loop seems to break if I try to count the amounts of same strings in the categories list. Does collections.Counter() modify the original categories object?

推荐答案

您似乎正在使用Python 3.

You seem to be using Python 3.

map 现在返回一个迭代器. collections.Counter(categories)耗尽迭代器，就像下面示例中的 list(m)一样

map now returns an iterator. collections.Counter(categories) exhausts the iterator, just like list(m) in the example below

In [3]: m = map(bool, [1, 2, 3, 4])

In [4]: list(m)
Out[4]: [True, True, True, True]

In [5]: list(m)
Out[5]: []

解决方案是构建序列，然后调用 collections.Counter .例如，可以使用 list :

The solution is to build a sequence before calling collections.Counter. For example, a list can be constructed using either list:

categories = list(map(operator.itemgetter(-1), sortedgroups))

或列表理解:

categories = [x[-1] for x in sortedgroups]

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