refactor: add / use 'Client._list_resource' method (googleapis#446)

* Adjust tests for 'Bucket.list_blobs' not to depend on anything but calling 'Client.list_blobs'.
* Provide explicit coverage for 'client._item_to_bucket' and 'client._item_to_hmack_key_metadata' helpers.
* Provide explicit coverage for 'bucket._item_to_notification'.

Toward googleapis#38

Author: tseaver

google/cloud/storage/bucket.py

@@ -17,14 +17,12 @@
 import base64
 import copy
 import datetime
-import functools
 import json
 import warnings
 
 import six
 from six.moves.urllib.parse import urlsplit
 
-from google.api_core import page_iterator
 from google.api_core import datetime_helpers
 from google.cloud._helpers import _datetime_to_rfc3339
 from google.cloud._helpers import _NOW
@@ -1392,14 +1390,8 @@ def list_notifications(
         """
         client = self._require_client(client)
         path = self.path + "/notificationConfigs"
-        api_request = functools.partial(
-            client._connection.api_request, timeout=timeout, retry=retry
-        )
-        iterator = page_iterator.HTTPIterator(
-            client=client,
-            api_request=api_request,
-            path=path,
-            item_to_value=_item_to_notification,
+        iterator = client._list_resource(
+            path, _item_to_notification, timeout=timeout, retry=retry,
         )
         iterator.bucket = self
         return iterator

google/cloud/storage/client.py

@@ -386,6 +386,31 @@ def _get_resource(
             _target_object=_target_object,
         )
 
+    def _list_resource(
+        self,
+        path,
+        item_to_value,
+        page_token=None,
+        max_results=None,
+        extra_params=None,
+        page_start=page_iterator._do_nothing_page_start,
+        timeout=_DEFAULT_TIMEOUT,
+        retry=DEFAULT_RETRY,
+    ):
+        api_request = functools.partial(
+            self._connection.api_request, timeout=timeout, retry=retry
+        )
+        return page_iterator.HTTPIterator(
+            client=self,
+            api_request=api_request,
+            path=path,
+            item_to_value=item_to_value,
+            page_token=page_token,
+            max_results=max_results,
+            extra_params=extra_params,
+            page_start=page_start,
+        )
+
     def _patch_resource(
         self,
         path,
@@ -1214,14 +1239,9 @@ def list_blobs(
             extra_params["userProject"] = bucket.user_project
 
         path = bucket.path + "/o"
-        api_request = functools.partial(
-            self._connection.api_request, timeout=timeout, retry=retry
-        )
-        iterator = page_iterator.HTTPIterator(
-            client=self,
-            api_request=api_request,
-            path=path,
-            item_to_value=_item_to_blob,
+        iterator = self._list_resource(
+            path,
+            _item_to_blob,
             page_token=page_token,
             max_results=max_results,
             extra_params=extra_params,
@@ -1328,18 +1348,14 @@ def list_buckets(
         if fields is not None:
             extra_params["fields"] = fields
 
-        api_request = functools.partial(
-            self._connection.api_request, retry=retry, timeout=timeout
-        )
-
-        return page_iterator.HTTPIterator(
-            client=self,
-            api_request=api_request,
-            path="/b",
-            item_to_value=_item_to_bucket,
+        return self._list_resource(
+            "/b",
+            _item_to_bucket,
             page_token=page_token,
             max_results=max_results,
             extra_params=extra_params,
+            timeout=timeout,
+            retry=retry,
         )
 
     def create_hmac_key(
@@ -1476,17 +1492,13 @@ def list_hmac_keys(
         if user_project is not None:
             extra_params["userProject"] = user_project
 
-        api_request = functools.partial(
-            self._connection.api_request, timeout=timeout, retry=retry
-        )
-
-        return page_iterator.HTTPIterator(
-            client=self,
-            api_request=api_request,
-            path=path,
-            item_to_value=_item_to_hmac_key_metadata,
+        return self._list_resource(
+            path,
+            _item_to_hmac_key_metadata,
             max_results=max_results,
             extra_params=extra_params,
+            timeout=timeout,
+            retry=retry,
         )
 
     def get_hmac_key_metadata(