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
