Add poll_until_no_more_changes to SubscriptionList (#35)

Refactor SubscriptionList class to include a new method poll_until_no_more_changes in subscription_list.py

macrobond_data_api/web/subscription_list.py

@@ -1,6 +1,6 @@
 import time
 from datetime import datetime, timezone, timedelta
-from typing import Sequence, List, Dict
+from typing import Sequence, List, Dict, Iterator
 
 from macrobond_data_api.common.types._parse_iso8601 import _parse_iso8601
 
@@ -13,7 +13,7 @@ class SubscriptionList:
 
     This class shouldn't be instantiated directly, but instead should be retrieved from a web client through `subscription_list`
 
-    Examples
+    Examples - for a program that polls for updates continuously
     --------
     ```python
     from datetime import datetime, timezone
@@ -26,15 +26,39 @@ class SubscriptionList:
             for key, date in result.items():
                 print(f'Series "{key}", last updated "{date}"')
     ```
+
+    Examples - for a program that retrieves all updates and then exits
+    --------
+    ```python
+    from datetime import datetime, timezone
+
+    # this should be the last time you polled the subscription list.
+    last_modified = previous_last_modified # or datetime.now(timezone.utc) if it's the first time you poll.
+
+    with WebClient() as api:
+        subscription_list = api.subscription_list(last_modified)
+        for result in subscription_list.poll_until_no_more_changes():
+            for key, date in result.items():
+                print(f'Series "{key}", last updated "{date}"')
+
+        # all done polling, so we can now update the last_modified time for next itme we poll.
+        last_modified = subscription_list.last_modified
+    ```
     """
 
     def __init__(self, session: Session, last_modified: datetime, poll_interval: timedelta = None):
         self._session = session
+
         self.last_modified = last_modified - timedelta(seconds=5)
         """
         Stores the date for when the subscription list was last modified.
         """
 
+        self.no_more_changes = False
+        """
+        An indicator that there are no changes at the moment.
+        """
+
         if poll_interval is None:
             poll_interval = timedelta(seconds=15)
 
@@ -122,7 +146,7 @@ def poll(self) -> Dict[str, datetime]:
 
         Returns
         -------
-        Optional[Dict[str, datetime]]
+        Dict[str, datetime]]
             A dictionary of primary keys that has been updated, and the corresponding last update date.
         """
         if not self._session._is_open:
@@ -136,12 +160,29 @@ def poll(self) -> Dict[str, datetime]:
             "v1/subscriptionlist/getupdates", params={"ifModifiedSince": self.last_modified.isoformat()}
         ).json()
 
-        if data["noMoreChanges"]:
+        self.no_more_changes = data["noMoreChanges"]
+        if self.no_more_changes:
             self._next_poll = datetime.now(timezone.utc) + self.poll_interval
 
         self.last_modified = _parse_iso8601(data["timeStampForIfModifiedSince"])
         return {entity["name"]: _parse_iso8601(entity["modified"]) for entity in data["entities"]}
 
+    def poll_until_no_more_changes(self) -> Iterator[Dict[str, datetime]]:
+        """
+        Polls for any changes on the series in the subscription list until thers no more changes.
+
+        Returns
+        -------
+        Iterator[Dict[str, datetime]]]
+            A Iterator of dictionarys of primary keys that has been updated, and the corresponding last update date.
+        """
+        while True:
+            changes = self.poll()
+            if len(changes) > 0:
+                yield changes
+            if self.no_more_changes:
+                break
+
     def _call(self, endpoint: str, keys: Sequence[str]) -> None:
         if not isinstance(keys, Sequence):
             raise TypeError("keys is not a sequence")