Forward mode differentiation for a constant

def _forward_mode(self, *args):
        """Forward mode differentiation for a constant"""
        # Evaluate inner function self.f
        X: np.ndarray
        dX: np.ndarray
        X, dX = self.f._forward_mode(*args)
        Y: np.ndarray
        dY: np.ndarray
        Y, dY = self.g._forward_mode(*args)
        # The function value
        val = self.func(X,Y)
        # The derivative (user is responsible for chain rule, etc.)
        diff = self.deriv(X,Y)
        return (val, diff)