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, a, b):   self.e1 = a   self.e2 = b  def d(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"))