<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def list_groups( self, name, children_of_group=None, ancestors_of_group=None, descendants_of_group=None, page_size=None, retry=google.api_core.gapic_v1.method.DEFAULT, timeout=google.api_core.gapic_v1.method.DEFAULT, metadata=None, ):
 """ Lists the existing groups. Example: Args: name (str):
 The project whose groups are to be listed. The format is ``"projects/{project_id_or_number}"``. children_of_group (str):
 A group name: ``"projects/{project_id_or_number}/groups/{group_id}"``. Returns groups whose ``parentName`` field contains the group name. If no groups have this parent, the results are empty. ancestors_of_group (str):
 A group name: ``"projects/{project_id_or_number}/groups/{group_id}"``. Returns groups that are ancestors of the specified group. The groups are returned in order, starting with the immediate parent and ending with the most distant ancestor. If the specified group has no immediate parent, the results are empty. descendants_of_group (str):
 A group name: ``"projects/{project_id_or_number}/groups/{group_id}"``. Returns the descendants of the specified group. This is a superset of the results returned by the ``childrenOfGroup`` filter, and includes children-of-children, and so forth. page_size (int):
 The maximum number of resources contained in the underlying API response. If page streaming is performed per- resource, this parameter does not affect the return value. If page streaming is performed per-page, this determines the maximum number of resources in a page. retry (Optional[google.api_core.retry.Retry]):
 A retry object used to retry requests. If ``None`` is specified, requests will not be retried. timeout (Optional[float]):
 The amount of time, in seconds, to wait for the request to complete. Note that if ``retry`` is specified, the timeout applies to each individual attempt. metadata (Optional[Sequence[Tuple[str, str]]]):
 Additional metadata that is provided to the method. Returns: A :class:`~google.gax.PageIterator` instance. By default, this is an iterable of :class:`~google.cloud.monitoring_v3.types.Group` instances. This object can also be configured to iterate over the pages of the response through the `options` parameter. Raises: google.api_core.exceptions.GoogleAPICallError: If the request failed for any reason. google.api_core.exceptions.RetryError: If the request failed due to a retryable error and retry attempts failed. ValueError: If the parameters are invalid. """

        if metadata is None:
            metadata = []
        metadata = list(metadata)
        # Wrap the transport method to add retry and timeout logic.
        if "list_groups" not in self._inner_api_calls:
            self._inner_api_calls[
                "list_groups"
            ] = google.api_core.gapic_v1.method.wrap_method(
                self.transport.list_groups,
                default_retry=self._method_configs["ListGroups"].retry,
                default_timeout=self._method_configs["ListGroups"].timeout,
                client_info=self._client_info,
            )

        # Sanity check: We have some fields which are mutually exclusive;
        # raise ValueError if more than one is sent.
        google.api_core.protobuf_helpers.check_oneof(
            children_of_group=children_of_group,
            ancestors_of_group=ancestors_of_group,
            descendants_of_group=descendants_of_group,
        )

        request = group_service_pb2.ListGroupsRequest(
            name=name,
            children_of_group=children_of_group,
            ancestors_of_group=ancestors_of_group,
            descendants_of_group=descendants_of_group,
            page_size=page_size,
        )
        if metadata is None:
            metadata = []
        metadata = list(metadata)
        try:
            routing_header = [("name", name)]
        except AttributeError:
            pass
        else:
            routing_metadata = google.api_core.gapic_v1.routing_header.to_grpc_metadata(
                routing_header
            )
            metadata.append(routing_metadata)

        iterator = google.api_core.page_iterator.GRPCIterator(
            client=None,
            method=functools.partial(
                self._inner_api_calls["list_groups"],
                retry=retry,
                timeout=timeout,
                metadata=metadata,
            ),
            request=request,
            items_field="group",
            request_token_field="page_token",
            response_token_field="next_page_token",
        )
        return iterator