<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _legacy_add_user(self, name, password, read_only, **kwargs):
 """Uses v1 system to add users, i.e. saving to system.users. """

        # Use a Collection with the default codec_options.
        system_users = self._collection_default_options('system.users')
        user = system_users.find_one({"user": name}) or {"user": name}
        if password is not None:
            user["pwd"] = auth._password_digest(name, password)
        if read_only is not None:
            user["readOnly"] = read_only
        user.update(kwargs)

        # We don't care what the _id is, only that it has one
        # for the replace_one call below.
        user.setdefault("_id", ObjectId())
        try:
            system_users.replace_one({"_id": user["_id"]}, user, True)
        except OperationFailure as exc:
            # First admin user add fails gle in MongoDB >= 2.1.2
            # See SERVER-4225 for more information.
            if 'login' in str(exc):
                pass
            # First admin user add fails gle from mongos 2.0.x
            # and 2.2.x.
            elif (exc.details and
                  'getlasterror' in exc.details.get('note', '')):
                pass
            else:
                raise