def _njacobian(self):
        """
        :return: The `numerical_jacobian` of the components of the ODEModel with
            regards to the dependent variables. This is not to be confused with
            the jacobian of the model as a whole, which is 2D and computed with
            regards to the dependent vars and the fit parameters, and the
            ODEModel still needs to integrated to compute that.

            Instead, this function is used by the ODE integrator, and is not
            meant for human consumption.
        """
        return [
            [sympy_to_py(
                    sympy.diff(expr, var), self.independent_vars + self.dependent_vars + self.params
                ) for var in self.dependent_vars
            ] for _, expr in self.items()
        ]