settings: Add CUSTOM_HOME_NOT_LOGGED_IN for type narrowing.

django-stubs dynamically collects the type annotation for us from the
settings, acknowledging mypy that `HOME_NOT_LOGGED_IN` is an
`Optional[str]`. Type narrowing with assertions does not play well with
the default value of the decorator, so we define the same setting
variable with a different name as `CUSTOM_HOME_NOT_LOGGED_IN` to bypass
this restriction.

Filed python/mypy#13087 to track this issue.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zproject/computed_settings.py

@@ -28,6 +28,7 @@ from .configured_settings import (
     AUTH_LDAP_SERVER_URI,
     AUTHENTICATION_BACKENDS,
     CAMO_URI,
+    CUSTOM_HOME_NOT_LOGGED_IN,
     DEBUG,
     DEBUG_ERROR_REPORTING,
     EMAIL_BACKEND,
@@ -38,7 +39,6 @@ from .configured_settings import (
     EXTERNAL_URI_SCHEME,
     EXTRA_INSTALLED_APPS,
     GOOGLE_OAUTH2_CLIENT_ID,
-    HOME_NOT_LOGGED_IN,
     INVITATION_LINK_VALIDITY_DAYS,
     IS_DEV_DROPLET,
     LOCAL_UPLOADS_DIR,
@@ -1039,10 +1039,14 @@ ONLY_LDAP = AUTHENTICATION_BACKENDS == ("zproject.backends.ZulipLDAPAuthBackend"
 USING_APACHE_SSO = "zproject.backends.ZulipRemoteUserBackend" in AUTHENTICATION_BACKENDS
 ONLY_SSO = AUTHENTICATION_BACKENDS == ("zproject.backends.ZulipRemoteUserBackend",)
 
-if HOME_NOT_LOGGED_IN is None:
-    if ONLY_SSO:
+if CUSTOM_HOME_NOT_LOGGED_IN is not None:
+    # We import this with a different name to avoid a mypy bug with
+    # type-narrowed default parameter values.
+    # https://github.com/python/mypy/issues/13087
+    HOME_NOT_LOGGED_IN = CUSTOM_HOME_NOT_LOGGED_IN
+elif ONLY_SSO:
     HOME_NOT_LOGGED_IN = "/accounts/login/sso/"
-    else:
+else:
     HOME_NOT_LOGGED_IN = "/login/"
 
 AUTHENTICATION_BACKENDS += ("zproject.backends.ZulipDummyBackend",)

zproject/default_settings.py

@@ -106,7 +106,7 @@ SOCIAL_AUTH_SYNC_CUSTOM_ATTRS_DICT: Dict[str, Dict[str, Dict[str, str]]] = {}
 
 # Other auth
 SSO_APPEND_DOMAIN: Optional[str] = None
-HOME_NOT_LOGGED_IN: Optional[str] = None
+CUSTOM_HOME_NOT_LOGGED_IN: Optional[str] = None
 
 VIDEO_ZOOM_CLIENT_ID = get_secret("video_zoom_client_id", development_only=True)
 VIDEO_ZOOM_CLIENT_SECRET = get_secret("video_zoom_client_secret")

zproject/dev_settings.py

@@ -79,7 +79,7 @@ WEB_PUBLIC_STREAMS_ENABLED = True
 INVITES_MIN_USER_AGE_DAYS = 0
 
 # Redirect to /devlogin/ by default in dev mode
-HOME_NOT_LOGGED_IN = "/devlogin/"
+CUSTOM_HOME_NOT_LOGGED_IN = "/devlogin/"
 LOGIN_URL = "/devlogin/"
 
 # For development convenience, configure the ToS/Privacy Policies