1 files changed, 44 insertions, 18 deletions
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index 95ee3f1b84..5f85653330 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -14,6 +14,7 @@
 
 import itertools
 import logging
+from collections import defaultdict
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import PresenceState
@@ -229,24 +230,49 @@ class SyncRestServlet(RestServlet):
         )
 
         logger.debug("building sync response dict")
-        return {
-            "account_data": {"events": sync_result.account_data},
-            "to_device": {"events": sync_result.to_device},
-            "device_lists": {
-                "changed": list(sync_result.device_lists.changed),
-                "left": list(sync_result.device_lists.left),
-            },
-            "presence": SyncRestServlet.encode_presence(sync_result.presence, time_now),
-            "rooms": {"join": joined, "invite": invited, "leave": archived},
-            "groups": {
-                "join": sync_result.groups.join,
-                "invite": sync_result.groups.invite,
-                "leave": sync_result.groups.leave,
-            },
-            "device_one_time_keys_count": sync_result.device_one_time_keys_count,
-            "org.matrix.msc2732.device_unused_fallback_key_types": sync_result.device_unused_fallback_key_types,
-            "next_batch": await sync_result.next_batch.to_string(self.store),
-        }
+
+        response: dict = defaultdict(dict)
+        response["next_batch"] = await sync_result.next_batch.to_string(self.store)
+
+        if sync_result.account_data:
+            response["account_data"] = {"events": sync_result.account_data}
+        if sync_result.presence:
+            response["presence"] = SyncRestServlet.encode_presence(
+                sync_result.presence, time_now
+            )
+
+        if sync_result.to_device:
+            response["to_device"] = {"events": sync_result.to_device}
+
+        if sync_result.device_lists.changed:
+            response["device_lists"]["changed"] = list(sync_result.device_lists.changed)
+        if sync_result.device_lists.left:
+            response["device_lists"]["left"] = list(sync_result.device_lists.left)
+
+        if sync_result.device_one_time_keys_count:
+            response[
+                "device_one_time_keys_count"
+            ] = sync_result.device_one_time_keys_count
+        if sync_result.device_unused_fallback_key_types:
+            response[
+                "org.matrix.msc2732.device_unused_fallback_key_types"
+            ] = sync_result.device_unused_fallback_key_types
+
+        if joined:
+            response["rooms"]["join"] = joined
+        if invited:
+            response["rooms"]["invite"] = invited
+        if archived:
+            response["rooms"]["leave"] = archived
+
+        if sync_result.groups.join:
+            response["groups"]["join"] = sync_result.groups.join
+        if sync_result.groups.invite:
+            response["groups"]["invite"] = sync_result.groups.invite
+        if sync_result.groups.leave:
+            response["groups"]["leave"] = sync_result.groups.leave
+
+        return response
 
     @staticmethod
     def encode_presence(events, time_now):