python实现计算倒数的方法

class Expr: 
 def __add__(self,other): 
  return Plus(self,other) 
 def __mul__(self,other): 
  return Times(self,other) 
class Int(Expr): 
 def __init__(self,n): 
  self.n = n 
 def d(self,v): 
  return Int(0) 
 def __str__(self): 
  return `self.n` 
class Var(Expr): 
 def __init__(self,var): 
  self.var = var 
 def d(self,v): 
  return Int(self.var == v and 1 or 0) 
 def __str__(self): 
  return self.var 
class Plus(Expr): 
 def __init__(self,a,b): 
  self.e1 = a 
  self.e2 = b 
 def d(self,v): 
  return Plus(self.e1.d(v),self.e2.d(v)) 
 def __str__(self): 
  return "(%s + %s)" % (self.e1,self.e2) 
class Times(Expr): 
 def __init__(self,v): 
  return Plus(Times(self.e1,self.e2.d(v)),Times(self.e1.d(v),self.e2))
 def __str__(self): 
  return "(%s * %s)" % (self.e1,self.e2) 
if __name__ == "__main__": 
 x = Var("x") 
 a = Var("a") 
 b = Var("b") 
 c = Var("c") 
 e = a * x * x + b * x + c 
 print "d(%s,x) = %s" % (e,e.d("x"))