Raise an error when trying to run a normal function with run_async=True

slack/rtm/client.py

@@ -496,6 +496,13 @@ async def _dispatch_event(self, event, data=None):
                         rtm_client=self, web_client=self._web_client, data=data
                     )
                 else:
+                    if self.run_async is True:
+                        raise client_err.SlackRequestError(
+                            f'The callback "{callback.__name__}" is NOT a coroutine. '
+                            "Running such with run_async=True is unsupported. "
+                            "Consider adding async/await to the method "
+                            "or going with run_async=False if your app is not really non-blocking."
+                        )
                     payload = {
                         "rtm_client": self,
                         "web_client": self._web_client,

tests/rtm/test_rtm_client_functional.py

@@ -6,6 +6,7 @@
 
 import slack
 import slack.errors as e
+from tests.helpers import async_test
 from tests.rtm.mock_web_api_server import setup_mock_web_api_server, cleanup_mock_web_api_server
 
 
@@ -240,3 +241,47 @@ def assert_on_close(**payload):
 
         self.client.start()
         self.assertTrue(self.called)
+
+    @async_test
+    async def test_run_async_valid(self):
+        client = slack.RTMClient(
+            token="xoxb-valid",
+            base_url="http://localhost:8765",
+            run_async=True,
+        )
+        client._web_client = slack.WebClient(
+            token="xoxb-valid",
+            base_url="http://localhost:8888",
+            run_async=True,
+        )
+        self.called = False
+
+        @slack.RTMClient.run_on(event="open")
+        async def handle_open_event(**payload):
+            self.called = True
+
+        client.start()  # intentionally no await here
+        await asyncio.sleep(3)
+        self.assertTrue(self.called)
+
+    @async_test
+    async def test_run_async_invalid(self):
+        client = slack.RTMClient(
+            token="xoxb-valid",
+            base_url="http://localhost:8765",
+            run_async=True,
+        )
+        client._web_client = slack.WebClient(
+            token="xoxb-valid",
+            base_url="http://localhost:8888",
+            run_async=True,
+        )
+        self.called = False
+
+        @slack.RTMClient.run_on(event="open")
+        def handle_open_event(**payload):
+            self.called = True
+
+        client.start()  # intentionally no await here
+        await asyncio.sleep(3)
+        self.assertFalse(self.called)