diff --git a/swh/objstorage/__init__.py b/swh/objstorage/__init__.py
index 8c530f7..16a9918 100644
--- a/swh/objstorage/__init__.py
+++ b/swh/objstorage/__init__.py
@@ -1,109 +1,109 @@
 # Copyright (C) 2016  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
 
 from swh.objstorage.objstorage import ObjStorage, ID_HASH_LENGTH  # noqa
 from swh.objstorage.backends.pathslicing import PathSlicingObjStorage
 from swh.objstorage.backends.in_memory import InMemoryObjStorage
 from swh.objstorage.api.client import RemoteObjStorage
 from swh.objstorage.multiplexer import (
     MultiplexerObjStorage, StripingObjStorage)
 from swh.objstorage.multiplexer.filter import add_filters
 from swh.objstorage.backends.seaweed import WeedObjStorage
 from swh.objstorage.backends.generator import RandomGeneratorObjStorage
 
 from typing import Callable, Dict, Union
 
 __all__ = ['get_objstorage', 'ObjStorage']
 
 
-_STORAGE_CLASSES: Dict[str, Union[type, Callable[..., type]]] = {
+_STORAGE_CLASSES = {
     'pathslicing': PathSlicingObjStorage,
     'remote': RemoteObjStorage,
     'memory': InMemoryObjStorage,
     'weed': WeedObjStorage,
     'random': RandomGeneratorObjStorage,
-}
+}  # type: Dict[str, Union[type, Callable[..., type]]]
 
 _STORAGE_CLASSES_MISSING = {
 }
 
 try:
     from swh.objstorage.backends.azure import (
         AzureCloudObjStorage,
         PrefixedAzureCloudObjStorage,
     )
     _STORAGE_CLASSES['azure'] = AzureCloudObjStorage
     _STORAGE_CLASSES['azure-prefixed'] = PrefixedAzureCloudObjStorage
 except ImportError as e:
     _STORAGE_CLASSES_MISSING['azure'] = e.args[0]
     _STORAGE_CLASSES_MISSING['azure-prefixed'] = e.args[0]
 
 try:
     from swh.objstorage.backends.rados import RADOSObjStorage
     _STORAGE_CLASSES['rados'] = RADOSObjStorage
 except ImportError as e:
     _STORAGE_CLASSES_MISSING['rados'] = e.args[0]
 
 try:
     from swh.objstorage.backends.libcloud import (
         AwsCloudObjStorage,
         OpenStackCloudObjStorage,
     )
     _STORAGE_CLASSES['s3'] = AwsCloudObjStorage
     _STORAGE_CLASSES['swift'] = OpenStackCloudObjStorage
 except ImportError as e:
     _STORAGE_CLASSES_MISSING['s3'] = e.args[0]
     _STORAGE_CLASSES_MISSING['swift'] = e.args[0]
 
 
 def get_objstorage(cls, args):
     """ Create an ObjStorage using the given implementation class.
 
     Args:
         cls (str): objstorage class unique key contained in the
             _STORAGE_CLASSES dict.
         args (dict): 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 cls in _STORAGE_CLASSES:
         return _STORAGE_CLASSES[cls](**args)
     else:
         raise ValueError('Storage class {} is not available: {}'.format(
                          cls,
                          _STORAGE_CLASSES_MISSING.get(cls, 'unknown name')))
 
 
 def _construct_filtered_objstorage(storage_conf, filters_conf):
     return add_filters(
         get_objstorage(**storage_conf),
         filters_conf
     )
 
 
 _STORAGE_CLASSES['filtered'] = _construct_filtered_objstorage
 
 
 def _construct_multiplexer_objstorage(objstorages):
     storages = [get_objstorage(**conf)
                 for conf in objstorages]
     return MultiplexerObjStorage(storages)
 
 
 _STORAGE_CLASSES['multiplexer'] = _construct_multiplexer_objstorage
 
 
 def _construct_striping_objstorage(objstorages):
     storages = [get_objstorage(**conf)
                 for conf in objstorages]
     return StripingObjStorage(storages)
 
 
 _STORAGE_CLASSES['striping'] = _construct_striping_objstorage