迭代协议

class Counter:
   def __init__(self, max_count):
       self.max_count = max_count
   
   def __iter__(self):
       self.current = 0
       return self
   
   def __next__(self):
       if self.current < self.max_count:
           self.current += 1
           return self.current
       raise StopIteration

调用协议

class Multiplier:
   def __init__(self, factor):
       self.factor = factor
   
   def __call__(self, x):
       return x * self.factor

double = Multiplier(2)
print(double(5))  # 10

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