'Return the field if it was found, None else'

def _feedUntil(self, field_name):

    if (self.__is_feeding or (self._field_generator and self._field_generator.gi_running)):
        self.warning(('Unable    to    get    %s    (and    generator    is    already    running)' % field_name))
        return None
    try:
        while True:
            field = self._field_generator.next()
            self._addField(field)
            if (field.name == field_name):
                return field
    except HACHOIR_ERRORS as err:
        if (self._fixFeedError(err) is False):
            raise
    except StopIteration:
        self._stopFeeding()
    return None
