Clarify list/set/dict/tuple comprehensions and enforce via flake8 (#6957)

Ensure good comprehension hygiene using flake8-comprehensions.

synapse/app/_base.py

@@ -141,7 +141,7 @@ def start_reactor(
 
 def quit_with_error(error_string):
     message_lines = error_string.split("\n")
-    line_length = max([len(l) for l in message_lines if len(l) < 80]) + 2
+    line_length = max(len(l) for l in message_lines if len(l) < 80) + 2
     sys.stderr.write("*" * line_length + "\n")
     for line in message_lines:
         sys.stderr.write(" %s\n" % (line.rstrip(),))

synapse/app/federation_sender.py

@@ -262,7 +262,7 @@ class FederationSenderHandler(object):
 
         # ... as well as device updates and messages
         elif stream_name == DeviceListsStream.NAME:
-            hosts = set(row.destination for row in rows)
+            hosts = {row.destination for row in rows}
             for host in hosts:
                 self.federation_sender.send_device_messages(host)
 
@@ -270,7 +270,7 @@ class FederationSenderHandler(object):
             # The to_device stream includes stuff to be pushed to both local
             # clients and remote servers, so we ignore entities that start with
             # '@' (since they'll be local users rather than destinations).
-            hosts = set(row.entity for row in rows if not row.entity.startswith("@"))
+            hosts = {row.entity for row in rows if not row.entity.startswith("@")}
             for host in hosts:
                 self.federation_sender.send_device_messages(host)
 

synapse/app/pusher.py

@@ -158,7 +158,7 @@ class PusherReplicationHandler(ReplicationClientHandler):
                 yield self.pusher_pool.on_new_notifications(token, token)
             elif stream_name == "receipts":
                 yield self.pusher_pool.on_new_receipts(
-                    token, token, set(row.room_id for row in rows)
+                    token, token, {row.room_id for row in rows}
                 )
         except Exception:
             logger.exception("Error poking pushers")