code: def deriv(self, t, y, data=None):
          " Calculate [dtheta/dt, d2theta/dt2] from [theta, dtheta/dt]."
          theta, dtheta_dt = y
          return np.array([dtheta_dt, - self.g_l * gv.sin(theta)]f deriv(self, t, y, data=None):
          " Calculate [dtheta/dt, d2theta/dt2] from [theta, dtheta/dt]."
          theta, dtheta_dt = y
          return np.array([dtheta_dt, - self.g_l * gv.sin(theta)])
docs: Calculate [dtheta/dt, d2theta/dt2] from [theta, dtheta/dt].