add common.add to add to repeated properties only if not already there

tried a bunch of other more sophisticated ways to do this in snarfed/webutil@280a93e092 (plus snarfed/webutil@6cb9c1f719), tried a custom repeated property class that acted like a set instead of a list, but couldn't get it to work reliably or compatibly enough.

common.py

@@ -223,3 +223,12 @@ def webmention_endpoint_cache_key(url):
 def webmention_discover(url, **kwargs):
     """Thin caching wrapper around :func:`web.discover`."""
     return webmention.discover(url, **kwargs)
+
+
+def add(seq, val):
+    """Appends val to seq if seq doesn't already contain it.
+
+    Useful for treating repeated ndb properties like sets instead of lists.
+    """
+    if val not in seq:
+        seq.append(val)

models.py

@@ -19,7 +19,7 @@ from oauth_dropins.webutil.models import ComputedJsonProperty, JsonProperty, Str
 from oauth_dropins.webutil.util import json_dumps, json_loads
 
 import common
-from common import base64_to_long, long_to_base64, redirect_unwrap
+from common import add, base64_to_long, long_to_base64, redirect_unwrap
 
 # maps string label to Protocol subclass. populated by ProtocolUserMeta.
 # seed with old and upcoming protocols that don't have their own classes (yet).
@@ -431,11 +431,9 @@ class Object(StringIdModel):
         assert '^^' not in self.key.id()
 
         if self.as1 and self.as1.get('objectType') == 'activity':
-            if 'activity' not in self.labels:
-                self.labels.append('activity')
-        else:
-            if 'activity' in self.labels:
-                self.labels.remove('activity')
+            add(self.labels, 'activity')
+        elif 'activity' in self.labels:
+            self.labels.remove('activity')
 
     def _post_put_hook(self, future):
         """Update :meth:`Protocol.load` cache."""

protocol.py

@@ -11,7 +11,7 @@ from granary import as1
 import werkzeug.exceptions
 
 import common
-from common import error
+from common import add, error
 from models import Follower, Object, PROTOCOLS, Target
 from oauth_dropins.webutil import util
 from oauth_dropins.webutil.util import json_dumps, json_loads
@@ -456,10 +456,9 @@ class Protocol:
             logger.info(f'Delivering to followers of {actor_id}')
             for f in Follower.query(Follower.to == from_cls.key_for(actor_id),
                                     Follower.status == 'active'):
-                if f.from_ not in obj.users:
-                    obj.users.append(f.from_)
-            if obj.users and 'feed' not in obj.labels:
-                obj.labels.append('feed')
+                add(obj.users, f.from_)
+            if obj.users:
+                add(obj.labels, 'feed')
 
         obj.put()
         return 'OK'
@@ -584,8 +583,7 @@ class Protocol:
             target = obj.undelivered.pop()
             domain = util.domain_from_link(target.uri, minimize=False)
             if g.user and domain == g.user.key.id():
-                if 'notification' not in obj.labels:
-                    obj.labels.append('notification')
+                add(obj.labels, 'notification')
 
             if (domain == util.domain_from_link(source, minimize=False)
                 and cls.LABEL != 'fake'):
@@ -611,8 +609,7 @@ class Protocol:
             try:
                 if recip.send(obj, target.uri):
                     obj.delivered.append(target)
-                    if 'notification' not in obj.labels:
-                        obj.labels.append('notification')
+                    add(obj.labels, 'notification')
             except BaseException as e:
                 code, body = util.interpret_http_exception(e)
                 if not code and not body:

web.py

@@ -21,6 +21,7 @@ from requests import HTTPError, RequestException, URLRequired
 from werkzeug.exceptions import BadGateway, BadRequest, HTTPException, NotFound
 
 import common
+from common import add
 from flask_app import app, cache
 from models import Follower, Object, PROTOCOLS, Target, User
 from protocol import Protocol
@@ -558,7 +559,7 @@ def webmention_task():
         source_protocol='web',
     )
     if not targets:
-        obj.labels.append('user')
+        add(obj.labels, 'user')
         obj.status = 'ignored'
         obj.put()
         return 'No targets', 204