variable_class_methods.py

    def calc_input_values(self):
        # calculate the real-valued input to operation
        return [v.value for v in self.input_variables]

    def forward(self):  
        # calculate the real-valued output of operation              
        return self.forward_op(*self.calc_input_values())

    @property
    def value(self):
        return self.forward()

    @value.setter
    def value(self, value):
        if callable(value):
            self.forward_op = value
        else:
            self.forward_op = lambda : value

        self.derivative_op = create_diff_fn(self.forward_op)