fix: reapply before_send on repeated setup calls

- Keep before_send in sync with the module-level configuration on every setup() call, not just during initial client creation.
- This fixes the case where the default client is initialized before posthog.before_send is assigned, causing later events to skip the callback unexpectedly.
- Add a regression test covering updates to before_send after client initialization.

Author: sipa-echo-zaoa

posthog/__init__.py

@@ -889,6 +889,7 @@ def setup() -> Client:
     # always set incase user changes it
     default_client.disabled = disabled
     default_client.debug = debug
+    default_client._set_before_send(before_send)
 
     return default_client
 

posthog/client.py

@@ -288,14 +288,7 @@ def __init__(
         else:
             self.log.setLevel(logging.WARNING)
 
-        if before_send is not None:
-            if callable(before_send):
-                self.before_send = before_send
-            else:
-                self.log.warning("before_send is not callable, it will be ignored")
-                self.before_send = None
-        else:
-            self.before_send = None
+        self._set_before_send(before_send)
 
         if self.enable_exception_autocapture:
             self.exception_capture = ExceptionCapture(self)
@@ -332,6 +325,16 @@ def __init__(
                 if send:
                     consumer.start()
 
+    def _set_before_send(self, before_send):
+        if before_send is not None:
+            if callable(before_send):
+                self.before_send = before_send
+            else:
+                self.log.warning("before_send is not callable, it will be ignored")
+                self.before_send = None
+        else:
+            self.before_send = None
+
     def new_context(self, fresh=False, capture_exceptions=True):
         """
         Create a new context for managing shared state. Learn more about [contexts](/docs/libraries/python#contexts).

posthog/test/test_before_send.py

@@ -248,3 +248,28 @@ def my_before_send(event):
             batch_data = mock_post.call_args[1]["batch"]
             enqueued_msg = batch_data[0]
             self.assertTrue(enqueued_msg["properties"]["module_level_before_send"])
+
+    def test_before_send_callback_updates_after_client_initialization(self):
+        def my_before_send(event):
+            event["properties"]["updated_after_init"] = True
+            return event
+
+        with mock.patch("posthog.client.batch_post") as mock_post:
+            posthog.api_key = FAKE_TEST_API_KEY
+            posthog.sync_mode = True
+
+            first_msg_uuid = posthog.capture("first_event", distinct_id="user1")
+
+            posthog.before_send = my_before_send
+            second_msg_uuid = posthog.capture("second_event", distinct_id="user1")
+
+            self.assertIsNotNone(first_msg_uuid)
+            self.assertIsNotNone(second_msg_uuid)
+            self.assertIs(posthog.default_client.before_send, my_before_send)
+
+            self.assertEqual(mock_post.call_count, 2)
+            first_batch = mock_post.call_args_list[0][1]["batch"]
+            second_batch = mock_post.call_args_list[1][1]["batch"]
+
+            self.assertNotIn("updated_after_init", first_batch[0]["properties"])
+            self.assertTrue(second_batch[0]["properties"]["updated_after_init"])