<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_instances(self, filter_="", page_size=None, page_token=None):
 """List instances for the client's project. See https://cloud.google.com/spanner/reference/rpc/google.spanner.admin.database.v1#google.spanner.admin.database.v1.InstanceAdmin.ListInstances :type filter_: string :param filter_: (Optional) Filter to select instances listed. See the ``ListInstancesRequest`` docs above for examples. :type page_size: int :param page_size: Optional. The maximum number of instances in each page of results from this request. Non-positive values are ignored. Defaults to a sensible value set by the API. :type page_token: str :param page_token: Optional. If present, return the next batch of instances, using the value, which must correspond to the ``nextPageToken`` value returned in the previous response. Deprecated: use the ``pages`` property of the returned iterator instead of manually passing the token. :rtype: :class:`~google.api_core.page_iterator.Iterator` :returns: Iterator of :class:`~google.cloud.spanner_v1.instance.Instance` resources within the client's project. """

        metadata = _metadata_with_prefix(self.project_name)
        path = "projects/%s" % (self.project,)
        page_iter = self.instance_admin_api.list_instances(
            path, page_size=page_size, metadata=metadata
        )
        page_iter.item_to_value = self._item_to_instance
        page_iter.next_page_token = page_token
        return page_iter