Clarify API boundaries and remove cast

This avoids needing to cast the settings by instead documenting the
expected bounds of the API offered.

Author: PeterJCLaw

django_lightweight_queue/app_settings.py

@@ -1,3 +1,11 @@
+"""
+Internal API facade over Django settings.
+
+This provides a typed interface over Django settings as well as handling default
+values for settings not set by users and supporting the addition of overrides in
+some management commands.
+"""
+
 from typing import Any, Dict, List, Union, Callable, Optional, Sequence
 
 from typing_extensions import Protocol
@@ -45,6 +53,11 @@ class Settings(Protocol):
     ATOMIC_JOBS: bool
 
 
+class LayeredSettings(Settings, Protocol):
+    def add_layer(self, layer: Settings) -> None:
+        ...
+
+
 class Defaults(Settings):
     WORKERS: Dict[QueueName, int] = {}
     BACKEND = 'django_lightweight_queue.backends.synchronous.SynchronousBackend'
@@ -84,7 +97,7 @@ def __getattr__(self, name: str) -> Any:
         raise AttributeError(f"Sorry, '{name}' is not a valid setting.")
 
 
-app_settings: Settings = AppSettings(layers=[
+app_settings: LayeredSettings = AppSettings(layers=[
     Defaults(),
     LongNameAdapter(django_settings),
 ])

django_lightweight_queue/utils.py

@@ -23,7 +23,7 @@
 
 from . import constants
 from .types import Logger, QueueName, WorkerNumber
-from .app_settings import Defaults, AppSettings, app_settings, LongNameAdapter
+from .app_settings import Defaults, app_settings, LongNameAdapter
 
 if TYPE_CHECKING:
     from .backends.base import BaseBackend
@@ -56,7 +56,7 @@ def with_prefix(names: Iterable[str]) -> Set[str]:
         unexpected_str = "' ,'".join(unexpected_names)
         warnings.warn("Ignoring unexpected setting(s) '{}'.".format(unexpected_str))
 
-    cast(AppSettings, app_settings).add_layer(LongNameAdapter(extra_settings))
+    app_settings.add_layer(LongNameAdapter(extra_settings))
 
 
 @lru_cache()