sessions: Fix type: ignore issues.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/cache_helpers.py

@@ -1,7 +1,6 @@
 # See https://zulip.readthedocs.io/en/latest/subsystems/caching.html for docs
 import datetime
 import logging
-from importlib import import_module
 from typing import Any, Callable, Dict, List, Tuple
 
 from django.conf import settings
@@ -22,6 +21,7 @@ from zerver.lib.cache import (
     user_profile_cache_key,
 )
 from zerver.lib.message import MessageDict
+from zerver.lib.sessions import session_engine
 from zerver.lib.users import get_all_api_keys
 from zerver.models import (
     Client,
@@ -75,7 +75,6 @@ def huddle_cache_items(items_for_remote_cache: Dict[str, Tuple[Huddle]],
                        huddle: Huddle) -> None:
     items_for_remote_cache[huddle_hash_cache_key(huddle.huddle_hash)] = (huddle,)
 
-session_engine = import_module(settings.SESSION_ENGINE)
 def session_cache_items(items_for_remote_cache: Dict[str, str],
                         session: Session) -> None:
     if settings.SESSION_ENGINE != "django.contrib.sessions.backends.cached_db":
@@ -83,7 +82,7 @@ def session_cache_items(items_for_remote_cache: Dict[str, str],
         # will be no store.cache_key attribute, and in any case we
         # don't need to fill the cache, since it won't exist.
         return
-    store = session_engine.SessionStore(session_key=session.session_key)  # type: ignore[attr-defined] # import_module
+    store = session_engine.SessionStore(session_key=session.session_key)
     items_for_remote_cache[store.cache_key] = store.decode(session.session_data)
 
 def get_active_realm_ids() -> List[int]:

zerver/lib/events.py

@@ -1,14 +1,11 @@
 # See https://zulip.readthedocs.io/en/latest/subsystems/events-system.html for
 # high-level documentation on how this system works.
 import copy
-from importlib import import_module
 from typing import Any, Callable, Dict, Iterable, Optional, Sequence, Set
 
 from django.conf import settings
 from django.utils.translation import ugettext as _
 
-session_engine = import_module(settings.SESSION_ENGINE)
-
 from version import API_FEATURE_LEVEL, ZULIP_VERSION
 from zerver.lib.actions import (
     default_stream_groups_to_dicts_sorted,

zerver/lib/sessions.py

@@ -1,17 +1,23 @@
 import logging
 from datetime import timedelta
 from importlib import import_module
-from typing import Any, List, Mapping, Optional
+from typing import Any, List, Mapping, Optional, Type, cast
 
 from django.conf import settings
 from django.contrib.auth import SESSION_KEY, get_user_model
+from django.contrib.sessions.backends.base import SessionBase
 from django.contrib.sessions.models import Session
 from django.utils.timezone import now as timezone_now
+from typing_extensions import Protocol
 
 from zerver.lib.timestamp import datetime_to_timestamp, timestamp_to_datetime
 from zerver.models import Realm, UserProfile, get_user_profile_by_id
 
-session_engine = import_module(settings.SESSION_ENGINE)
+
+class SessionEngine(Protocol):
+    SessionStore: Type[SessionBase]
+
+session_engine = cast(SessionEngine, import_module(settings.SESSION_ENGINE))
 
 def get_session_dict_user(session_dict: Mapping[str, int]) -> Optional[int]:
     # Compare django.contrib.auth._get_user_session_key
@@ -28,7 +34,7 @@ def user_sessions(user_profile: UserProfile) -> List[Session]:
             if get_session_user(s) == user_profile.id]
 
 def delete_session(session: Session) -> None:
-    session_engine.SessionStore(session.session_key).delete()  # type: ignore[attr-defined] # import_module
+    session_engine.SessionStore(session.session_key).delete()
 
 def delete_user_sessions(user_profile: UserProfile) -> None:
     for session in Session.objects.all():