def __setitem__(self, key, value):
        # To conform with the mapping protocol, overwrites existing values in
        # the section.

        # XXX this is not atomic if read_dict fails at any point. Then again,
        # no update method in configparser is atomic in this implementation.
        if key == self.default_section:
            self._defaults.clear()
        elif key in self._sections:
            self._sections[key].clear()
        self.read_dict({key: value})