How to use the impedance.fitting.check_and_eval function in impedance

def get_param_names(self):
        """ Converts circuit string to names and units """

        # parse the element names from the circuit string
        names = self.circuit.replace('p', '').replace('(', '').replace(')', '')
        names = names.replace(',', '-').replace(' ', '').split('-')

        full_names, all_units = [], []
        for name in names:
            num_params = check_and_eval(name[0]).num_params
            units = check_and_eval(name[0]).units
            if num_params > 1:
                for j in range(num_params):
                    full_name = '{}_{}'.format(name, j)
                    if full_name not in self.constants.keys():
                        full_names.append(full_name)
                        all_units.append(units[j])
            else:
                if name not in self.constants.keys():
                    full_names.append(name)
                    all_units.append(units[0])

        return full_names, all_units

def get_param_names(self):
        """ Converts circuit string to names and units """

        # parse the element names from the circuit string
        names = self.circuit.replace('p', '').replace('(', '').replace(')', '')
        names = names.replace(',', '-').replace(' ', '').split('-')

        full_names, all_units = [], []
        for name in names:
            num_params = check_and_eval(name[0]).num_params
            units = check_and_eval(name[0]).units
            if num_params > 1:
                for j in range(num_params):
                    full_name = '{}_{}'.format(name, j)
                    if full_name not in self.constants.keys():
                        full_names.append(full_name)
                        all_units.append(units[j])
            else:
                if name not in self.constants.keys():
                    full_names.append(name)
                    all_units.append(units[0])

        return full_names, all_units

def __str__(self):
        """ Defines the pretty printing of the circuit"""

        to_print = '\n'
        if self.name is not None:
            to_print += 'Name: {}\n'.format(self.name)
        to_print += 'Circuit string: {}\n'.format(self.circuit)
        to_print += "Fit: {}\n".format(self._is_fit())

        if len(self.constants) > 0:
            to_print += '\nConstants:\n'
            for name, value in self.constants.items():
                units = check_and_eval(name[0]).units
                if '_' in name:
                    unit = units[int(name.split('_')[-1])]
                else:
                    unit = units[0]
                to_print += '  {:>5} = {:.2e} [{}]\n'.format(name, value, unit)

        names, units = self.get_param_names()
        to_print += '\nInitial guesses:\n'
        for name, unit, param in zip(names, units, self.initial_guess):
            to_print += '  {:>5} = {:.2e} [{}]\n'.format(name, param, unit)
        if self._is_fit():
            params, confs = self.parameters_, self.conf_
            to_print += '\nFit parameters:\n'
            for name, unit, param, conf in zip(names, units, params, confs):
                to_print += '  {:>5} = {:.2e}'.format(name, param)
                to_print += '  (+/- {:.2e}) [{}]\n'.format(conf, unit)