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 = g
def __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 cow
dog = animal(‘flopp’, 4) #we can make many animals
print ‘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 cow
cow(‘gras’)
print cow.stomach
#we give food to dog
dog(‘bone’)
dog(‘beef’)
print dog.stomach
#what comes inn most come out
print cow.poop()
print cow.stomach #empty stomach
”’–>output
we have 2 animales a cow name king and dog named flopp,both have 4 legs
a animal named king
[‘gras’]
[‘bone’, ‘beef’]
gras
[]
”’