<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save_model(self, fname):
 """ Save the model to a file. The model is saved in an XGBoost internal binary format which is universal among the various XGBoost interfaces. Auxiliary attributes of the Python Booster object (such as feature_names) will not be saved. To preserve all attributes, pickle the Booster object. Parameters fname : string Output file name """

        if isinstance(fname, STRING_TYPES):  # assume file name
            _check_call(_LIB.XGBoosterSaveModel(self.handle, c_str(fname)))
        else:
            raise TypeError("fname must be a string")