model: Add stream and subscription property accessor functions.

This commit adds stream and subscription property accessor functions to
limit the mutability of stream/subscription properties where it is not
needed.

Author: theViz343

zulipterminal/model.py

@@ -28,7 +28,7 @@
 
 import zulip
 from bs4 import BeautifulSoup
-from typing_extensions import TypedDict
+from typing_extensions import Literal, TypedDict
 
 from zulipterminal import unicode_emojis
 from zulipterminal.api_types import (
@@ -1297,6 +1297,53 @@ def _get_all_stream_ids(self) -> List[int]:
         id_list.extend(stream_id for stream_id in self._never_subscribed_streams)
         return id_list
 
+    STREAM_KEYS = Literal[
+        "stream_id",
+        "name",
+        "description",
+        "rendered_description",
+        "date_created",
+        "invite_only",
+        "subscribers",
+        "is_announcement_only",
+        "stream_post_policy",
+        "is_web_public",
+        "message_retention_days",
+        "history_public_to_subscribers",
+        "first_message_id",
+        "stream_weekly_traffic",
+    ]
+    SUBSCRIPTION_KEYS = Literal[
+        STREAM_KEYS,
+        "desktop_notifications",
+        "email_notifications",
+        "wildcard_mentions_notify",
+        "push_notifications",
+        "audible_notifications",
+        "pin_to_top",
+        "email_address",
+        "is_muted",
+        "color",
+    ]
+
+    def stream_property(
+        self, stream_id: int, property: STREAM_KEYS
+    ) -> Optional[Union[int, str, bool, List[int]]]:
+        return self._get_stream_from_id(stream_id)[property]
+
+    def subscription_property(
+        self, stream_id: int, property: SUBSCRIPTION_KEYS
+    ) -> Optional[Union[int, str, bool, List[int]]]:
+        subscription = self._get_stream_from_id(stream_id)
+        if property in subscription:
+            subscription = cast(Subscription, subscription)
+            return subscription[property]
+        else:
+            # stream_id is not a subscribed stream.
+            raise RuntimeError(
+                f"Stream with id={stream_id} does not have '{property}' property!"
+            )
+
     def _subscribe_to_streams(self, subscriptions: List[Subscription]) -> None:
         def make_reduced_stream_data(stream: Subscription) -> StreamData:
             # stream_id has been changed to id.