名为__call__的特殊方法

通过名为__call__的特殊方法，可以使类的实例能够像普通的Python函数那样得到调用。

current = {'green':12,'blue':3}

increments = [

('red',5),

('blue',17),

('orange',9),

]

class BetterCountMissing():

        def __init__(self):

                self.added = 0

        def __call__(self):

                self.added += 1

                return 0

counter = BetterCountMissing()

result = defaultdict(counter, current)

for key, amount in increments:

        result[key] += amount

assert counter.added == 2