new_contents: #!/usr/bin/env python
  
  import math
  import numpy as np
  
  
  class Op(object):
    def __init__(self):
      pass
  
    def forward(self):
      pass
  
    def grad(self):
      pass
  
  
  class ConstantOp(Op):
    def __init__(self, x):
      self.x = x
  
    def forward(self):
      return self.x
  
    def grad(self):
      return 0
  
  
  class CoefficientOp(Op):
    def __init__(self, x):
      self.x = x
  
    def forward(self):
      return self.x
  
    def grad(self):
      return self.x
  
  
  class VariableOp(Op):
    def __init__(self, x):
      self.x = x
  
    def forward(self):
      return self.x
  
    def grad(self):
      return 1
  
  
  def test_VariableOp():
    x = 10
    variable = VariableOp(x)
    print("X: {}, forward: {}, grad: {}".format(
        x, variable.forward(), variable.grad()))
  
  
  class SquareOp(Op):
    def __init__(self, x):
      self.x = x
  
    def forward(self):
      return pow(self.x, 2)
  
    def grad(self):
      return 2 * self.x
  
  
  def test_SquareOp():
    x = 10
    variable = SquareOp(x)
    print("X: {}, forward: {}, grad: {}".format(
        x, variable.forward(), variable.grad()))
  
  
  class CubicOp(Op):
    def __init__(self, x):
      self.x = x
  
    def forward(self):
      return math.pow(self.x, 3)
  
    def grad(self):
      return 3 * math.pow(self.x, 2)
  
  
  def test_CubicOp():
    x = 10
    variable = CubicOp(x)
    print("X: {}, forward: {}, grad: {}".format(
        x, variable.forward(), variable.grad()))
  
  
  class AddOp(Op):
    def __init__(self, *ops):
      self.ops = ops
  
    def forward(self):
      result = 0
      for op in self.ops:
        result += op.forward()
      return result
  
    def grad(self):
      result = 0
      for op in self.ops:
        result += op.grad()
      return result
  
  
  class MultipleOp(Op):
    def __init__(self, *ops):
      self.ops = ops
  
    def forward(self):
      result = 1
      for op in self.ops:
        result *= op.forward()
      return result
  
    def grad(self):
      result = 1
      for op in self.ops:
        result *= op.grad()
      return result
  
  
  def main():
    x = 1
  
    # y = 3 * x**3 + 2 * x**2 + x + 10
    # y' =  9 * x**2 + 4 * x + 1
  
    first_item = MultipleOp(CoefficientOp(3), CubicOp(x))
    second_item = MultipleOp(CoefficientOp(2), SquareOp(x))
    third_item = VariableOp(x)
    forth_item = ConstantOp(10)
    y = AddOp(AddOp(first_item, second_item), third_item, forth_item)
  
    # Should be "X: 1, forward: 16.0, grad: 14.0"
    print("X: {}, forward: {}, grad: {}".format(x, y.forward(), y.grad()))
  
  
  if __name__ == "__main__":
    main()
  
subject: Implement auto gradient