diff --git a/swh/objstorage/factory.py b/swh/objstorage/factory.py
index 5843482..aa21e8d 100644
--- a/swh/objstorage/factory.py
+++ b/swh/objstorage/factory.py
@@ -1,97 +1,98 @@
 # Copyright (C) 2016-2022  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import importlib
 import warnings
 
+from swh.objstorage.interface import ObjStorageInterface
 from swh.objstorage.multiplexer import MultiplexerObjStorage, StripingObjStorage
 from swh.objstorage.multiplexer.filter import add_filters
 from swh.objstorage.objstorage import ObjStorage
 
 __all__ = ["get_objstorage", "ObjStorage"]
 
 
 OBJSTORAGE_IMPLEMENTATIONS = {
     "pathslicing": ".backends.pathslicing.PathSlicingObjStorage",
     "remote": ".api.client.RemoteObjStorage",
     "memory": ".backends.in_memory.InMemoryObjStorage",
     "seaweedfs": ".backends.seaweedfs.SeaweedFilerObjStorage",
     "random": ".backends.generator.RandomGeneratorObjStorage",
     "http": ".backends.http.HTTPReadOnlyObjStorage",
     "noop": ".backends.noop.NoopObjStorage",
     "azure": ".backends.azure.AzureCloudObjStorage",
     "azure-prefixed": ".backends.azure.PrefixedAzureCloudObjStorage",
     "s3": ".backends.libcloud.AwsCloudObjStorage",
     "swift": ".backends.libcloud.OpenStackCloudObjStorage",
     "winery": ".backends.winery.WineryObjStorage",
 }
 
 
-def get_objstorage(cls: str, args=None, **kwargs):
+def get_objstorage(cls: str, args=None, **kwargs) -> ObjStorageInterface:
     """Create an ObjStorage using the given implementation class.
 
     Args:
         cls: objstorage class unique key contained in the
             _STORAGE_CLASSES dict.
         kwargs: arguments for the required class of objstorage
                 that must match exactly the one in the `__init__` method of the
                 class.
     Returns:
         subclass of ObjStorage that match the given `storage_class` argument.
     Raises:
         ValueError: if the given storage class is not a valid objstorage
             key.
     """
     if args is not None:
         warnings.warn(
             'Explicit "args" key is deprecated for objstorage initialization, '
             "use class arguments keys directly instead.",
             DeprecationWarning,
         )
         # TODO: when removing this, drop the "args" backwards compatibility
         # from swh.objstorage.api.server configuration checker
         kwargs = args
 
     class_path = OBJSTORAGE_IMPLEMENTATIONS.get(cls)
     if class_path is None:
         raise ValueError(
             "Unknown storage class `%s`. Supported: %s"
             % (cls, ", ".join(OBJSTORAGE_IMPLEMENTATIONS))
         )
 
     if "." in class_path:
         (module_path, class_name) = class_path.rsplit(".", 1)
         try:
             module = importlib.import_module(module_path, package=__package__)
         except ImportError as e:
             raise ValueError(f"Storage class {cls} is not available: {e.args[0]}")
         ObjStorage = getattr(module, class_name)
     else:
         ObjStorage = globals()[class_path]
 
     return ObjStorage(**kwargs)
 
 
 def _construct_filtered_objstorage(storage_conf, filters_conf):
     return add_filters(get_objstorage(**storage_conf), filters_conf)
 
 
 OBJSTORAGE_IMPLEMENTATIONS["filtered"] = "_construct_filtered_objstorage"
 
 
 def _construct_multiplexer_objstorage(objstorages):
     storages = [get_objstorage(**conf) for conf in objstorages]
     return MultiplexerObjStorage(storages)
 
 
 OBJSTORAGE_IMPLEMENTATIONS["multiplexer"] = "_construct_multiplexer_objstorage"
 
 
 def _construct_striping_objstorage(objstorages):
     storages = [get_objstorage(**conf) for conf in objstorages]
     return StripingObjStorage(storages)
 
 
 OBJSTORAGE_IMPLEMENTATIONS["striping"] = "_construct_striping_objstorage"