Support stopping propagation of events (#622)

telethon/events/__init__.py

@@ -873,3 +873,26 @@ class MessageChanged(_EventBuilder):
             self.edited = bool(edit_msg)
             self.deleted = bool(deleted_ids)
             self.deleted_ids = deleted_ids or []
+
+
+class StopPropagation(Exception):
+    """
+    If this Exception is found to be raised in any of the handlers for a
+    given update, it will stop the execution of all other registered
+    event handlers in the chain.
+    Think of it like a ``StopIteration`` exception in a for loop.
+
+    Example usage:
+    ```
+    @client.on(events.NewMessage)
+    def delete(event):
+        event.delete()
+        # Other handlers won't have an event to work with
+        raise StopPropagation
+
+    @client.on(events.NewMessage)
+    def _(event):
+        # Will never be reached, because it is the second handler in the chain.
+        pass
+    ```
+    """

telethon/telegram_client.py

@@ -1885,7 +1885,15 @@ class TelegramClient(TelegramBareClient):
             event = builder.build(update)
             if event:
                 event._client = self
-                callback(event)
+                try:
+                    callback(event)
+                except events.StopPropagation:
+                    __log__.debug(
+                        "Event handler '{}' stopped chain of "
+                        "propagation for event {}."
+                        .format(callback.__name__, type(event).__name__)
+                    )
+                    break
 
     def add_event_handler(self, callback, event=None):
         """