class Value:

• “class” is the python command to create a new kind of objects.
• In this case, the name of the new kind of objects is “Value”
• When a new kind of object is defined, two kind of properties must be defined:
  a set of data item associated with each instance of the object
  a set of python functions (called “methods”) that operate on the data items associated with an instance of the object

  def __init__(self, data, _children=(), _op='', label=''):

• Define the ‘__init__’ method for the Value class
• This method is called each time a new instance of the Value object is created

    self.data = data
    self.grad = 0.0
    self._backward = lambda: None
    self._prev = set(_children)
    self._op = _op
    self.label = label

  def __repr__(self):
    return f"Value(data={self.data})"
  
  def __add__(self, other):
    out = Value(self.data + other.data, (self, other), '+')
    
    def _backward():
      self.grad += 1.0 * out.grad
      other.grad += 1.0 * out.grad
    out._backward = _backward
    
    return out

  def __mul__(self, other):
    out = Value(self.data * other.data, (self, other), '*')
    
    def _backward():
      self.grad += other.data * out.grad
      other.grad += self.data * out.grad
    out._backward = _backward
      
    return out
  
  def tanh(self):
    x = self.data
    t = (math.exp(2*x) - 1)/(math.exp(2*x) + 1)
    out = Value(t, (self, ), 'tanh')
    
    def _backward():
      self.grad += (1 - t**2) * out.grad
    out._backward = _backward
    
    return out
  
  def backward(self):
    
    topo = []
    visited = set()
    def build_topo(v):
      if v not in visited:
        visited.add(v)
        for child in v._prev:
          build_topo(child)
        topo.append(v)
    build_topo(self)
    
    self.grad = 1.0
    for node in reversed(topo):
      node._backward()


a = Value(2.0, label='a')
b = Value(-3.0, label='b')
c = Value(10.0, label='c')
e = a*b; e.label = 'e'
d = e + c; d.label = 'd'
f = Value(-2.0, label='f')
L = d * f; L.label = 'L'
print(L)