python 计算倒数代码

python 计算倒数代码

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"))