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pythonpython-2.7counter

Summing list of counters in python

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?

Solution

  • 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})