python中有一个有趣的语法，只要定义类型的时候，实现__call__函数，这个类型就成为可调用的。
换句话说，我们可以把这个类型的对象当作函数来使用，相当于 重载了括号运算符。
class g_dpm(object):def __init__(self, g):self.g = gdef __call__(self, t):return (self.g*t**2)/2
计算地球场景的时候，我们就可以令e_dpm = g_dpm(9.8)，s = e_dpm(t)。
class animal(object): def __init__(self, name, legs): self.name = name self.legs = legs self.stomach = [] def __call__(self,food): self.stomach.append(food) def poop(self): if len(self.stomach) > 0: return self.stomach.pop(0) def __str__(self): return 'a animal named %s' % (self.name) cow = animal('king', 4) #we make a cowdog = animal('flopp', 4) #we can make many animalsprint 'we have 2 animales a cow name %s and dog named %s,both have %s legs' % (cow.name, dog.name, cow.legs)print cow #here __str__ metod work #we give food to cowcow('gras')print cow.stomach #we give food to dogdog('bone')dog('beef')print dog.stomach #what comes inn most come outprint cow.poop()print cow.stomach #empty stomach '''-->outputwe have 2 animales a cow name king and dog named flopp,both have 4 legsa animal named king['gras']['bone', 'beef']gras[]'''