python初始为0的计数器,python中的计数器汇总

I am looking to sum a list of counters in python. For example to sum:

counter_list = [Counter({"a":1, "b":2}), Counter({"b":3, "c":4})]

to give Counter({'b': 5, 'c': 4, 'a': 1})

I can get the following code to do the summation:

counter_master = Counter()

for element in counter_list:

counter_master = counter_master + element

But I am confused as to why counter_master = sum(counter_list) results in the error TypeError: unsupported operand type(s) for +: 'int' and 'Counter' ? Given it is possible to add counters together, why is it not possible to sum them?

解决方案

The sum function has the optional start argument which defaults to 0. Quoting the linked page:

sum(iterable[, start])

Sums start and the items of an iterable from left to right and returns

the total

Set start to (empty) Counter object to avoid the TypeError:

In [5]: sum(counter_list, Counter())

Out[5]: Counter({'b': 5, 'c': 4, 'a': 1})