def reset_option(self, key, subkey):
if not self.open:<EOL><INDENT>return<EOL><DEDENT>key, subkey = _lower_keys(key, subkey)<EOL>_entry_must_exist(self.gc, key, subkey)<EOL>df = self.gc[(self.gc["<STR_LIT>"] == key) & (self.gc["<STR_LIT>"] == subkey)]<EOL>if df["<STR_LIT>"].values[<NUM_LIT:0>]:<EOL><INDENT>raise ValueError("<STR_LIT>".format(key, subkey))<EOL><DEDENT>val = df["<STR_LIT:default>"].values[<NUM_LIT:0>]<EOL>self.gc.loc[<EOL>(self.gc["<STR_LIT>"] == key) &<EOL>(self.gc["<STR_LIT>"] == subkey), "<STR_LIT:value>"] = val<EOL>
Resets a single option to the default values.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.
            :ValueError: If the targeted obtion is locked.