Retrives credentials from a file. Defaults to the user config
        directory

def _retrieve(self, config_file=None):
        """Retrives credentials from a file. Defaults to the user config
        directory"""
        with open(config_file or _DEFAULT_PATH, 'r') as f:
            creds = json.load(f)
        self._key = creds.get('key')
        self._base_url = creds.get('base_url')
        self._username = creds.get('username')