collections.Counter 计数器求和的计数器加法错误

Question

假设我们有如下代码片段：

from collections import namedtuple, Counter

Experiment = namedtuple('Experiment', ['day', 'distribution'])
LabResults = namedtuple('LabResults', ['lab_name', 'experiments_data'])

lab_results_list_good = [
    LabResults(
        'Lab A', 
        [
            Experiment('first_day',  Counter('abc')),
            Experiment('second_day', Counter(''))
        ],
    ),
    LabResults(
        'Lab B',
        [
            Experiment('first_day', Counter('')),
            Experiment('second_day', Counter('a')),
        ]
    )
]

lab_results_list_bad = [
    LabResults(
        'Lab A', 
        [
            Experiment('first_day', Counter('abc')),
            Experiment('second_day', Counter('def'))
        ],
    ),
    LabResults(
        'Lab B',
        [
            Experiment('first_day', Counter('ghi')),
        ]
    )
]


def merge_distributions(lab_results_list):
    day_to_distribution_report = Counter()
    for lab_result in lab_results_list:

        curr_day_to_dist = Counter({exp.day: exp.distribution for exp in lab_result.experiments_data})
        day_to_distribution_report += curr_day_to_dist
    return day_to_distribution_report

第一种情况工作正常，第二种情况不工作：

In [2]: print merge_distributions(lab_results_list_good)
Counter({'first_day': Counter({'a': 1, 'c': 1, 'b': 1}), 'second_day': Counter({'a': 1})})

In [3]: print merge_distributions(lab_results_list_bad)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-969bd085f2e5> in <module>()
----> 1 print merge_distributions(lab_results_list_bad)

<ipython-input-1-0bdc39e8c846> in merge_distributions(lab_results_list)
     42
     43         curr_day_to_dist = Counter({exp.day: exp.distribution for exp in lab_result.experiments_data})
---> 44         day_to_distribution_report += curr_day_to_dist
     45     return day_to_distribution_report

/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/collections.pyc in __add__(self, other)
    642         result = Counter()
    643         for elem, count in self.items():
--> 644             newcount = count + other[elem]
    645             if newcount > 0:
    646                 result[elem] = newcount

TypeError: unsupported operand type(s) for +: 'Counter' and 'int'

In Python 2 只在第二种情况下不起作用，在 Python 3 中两者都不起作用。

所以我的问题是合并计数器的计数器是一个非常糟糕的主意，因为 python 3 两种情况都失败了运行？这似乎是一个错误，因为 Counter 对象支持添加。

编辑我找到了 merge_distribution 函数的快速修复：

def merge_distributions(lab_results_list):
    day_range = {exp.day for lab_result in lab_results_list for exp in lab_result.experiments_data}
    day_to_distribution_report = Counter()
    for lab_result in lab_results_list:

        curr_day_to_dist = Counter({exp.day: exp.distribution for exp in lab_result.experiments_data})
        map(lambda day: curr_day_to_dist.setdefault(day, Counter()), day_range)
        day_to_distribution_report += curr_day_to_dist
    return day_to_distribution_report

为什么 python 来源默认不做类似的事情？

Answer 1

如果计数器没有属性，则默认为 0，而不是 Counter()

>>> from collections import Counter
>>> c = Counter({
...     'a': Counter(), 'b': Counter(),
... })
>>> d = Counter({
...     'a': Counter()
... })
>>> c['b']
Counter()
>>> d['b']
0
>>> c['b'] + d['b']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'Counter' and 'int'
>>>

这里有一个DefaultCounter实现：

from collections import Counter


class DefaultCounter(Counter):

    def __add__(self, other):
        if not isinstance(other, Counter):
            if other == 0:
                other = Counter()
            else:
                return NotImplemented
        result = Counter()
        for elem, count in self.items():
            newcount = count + other[elem]
            if newcount > 0:
                result[elem] = newcount
        for elem, count in other.items():
            if elem not in self and count > 0:
                result[elem] = count
        return result

>>> from m import DefaultCounter
>>> a = DefaultCounter({'a': DefaultCounter(), 'b': DefaultCounter()})
>>> b = DefaultCounter({'a': DefaultCounter()})
>>> 
>>> a + b
Counter({'a': Counter(), 'b': Counter()})
# not raise anymore. ^_^

collections.Counter 计数器求和的计数器加法错误

collections.Counter addition error with counter of counters summation

python

collections

counter

python-2.7

python-3.x