diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -178,8 +178,8 @@
         partition_id: int,
         nb_partitions: int,
         limit: int = 1000,
-        page_token: str = None,
-    ):
+        page_token: Optional[str] = None,
+    ) -> Dict[str, Any]:
         if limit is None:
             raise StorageArgumentException("limit should not be None")
 
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -292,8 +292,8 @@
         partition_id: int,
         nb_partitions: int,
         limit: int = 1000,
-        page_token: str = None,
-    ):
+        page_token: Optional[str] = None,
+    ) -> Dict[str, Any]:
         if limit is None:
             raise StorageArgumentException("limit should not be None")
         (start, end) = get_partition_bounds_bytes(
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -208,8 +208,8 @@
         partition_id: int,
         nb_partitions: int,
         limit: int = 1000,
-        page_token: str = None,
-    ):
+        page_token: Optional[str] = None,
+    ) -> Dict[str, Any]:
         """Splits contents into nb_partitions, and returns one of these based on
         partition_id (which must be in [0, nb_partitions-1])
 
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -305,8 +305,8 @@
         partition_id: int,
         nb_partitions: int,
         limit: int = 1000,
-        page_token: str = None,
-    ):
+        page_token: Optional[str] = None,
+    ) -> Dict[str, Any]:
         if limit is None:
             raise StorageArgumentException("limit should not be None")
         (start, end) = get_partition_bounds_bytes(