--tags: Log a warning if no tasks remain on a User/TaskSet after filt…

…ering

locust/test/test_main.py

@@ -1077,6 +1077,38 @@ def test_error_when_locustfiles_directory_is_empty(self):
             self.assertIn(f"Could not find any locustfiles in directory '{temp_dir}'", stderr)
             self.assertEqual(1, proc.returncode)
 
+    def test_error_when_no_tasks_match_tags(self):
+        content = """
+from locust import HttpUser, TaskSet, task, constant, LoadTestShape, tag
+class MyUser(HttpUser):
+    host = "http://127.0.0.1:8089"
+    wait_time = constant(1)
+    @tag("tag1")
+    @task
+    def task1(self):
+        print("task1")
+    """
+        with mock_locustfile(content=content) as mocked:
+            proc = subprocess.Popen(
+                [
+                    "locust",
+                    "-f",
+                    mocked.file_path,
+                    "--headless",
+                    "-t",
+                    "1",
+                    "--tags",
+                    "tag2",
+                ],
+                stdout=PIPE,
+                stderr=PIPE,
+                text=True,
+            )
+            stdout, stderr = proc.communicate()
+            self.assertIn("MyUser had no tasks left after filtering", stderr)
+            self.assertIn("No tasks defined on MyUser", stderr)
+            self.assertEqual(1, proc.returncode)
+
 
 class DistributedIntegrationTests(ProcessIntegrationTest):
     def test_expect_workers(self):

locust/user/task.py

@@ -208,6 +208,8 @@ def filter_tasks_by_tags(
         checked[task] = passing
 
     task_holder.tasks = new_tasks
+    if not new_tasks:
+        logging.warning(f"{task_holder.__name__} had no tasks left after filtering, instantiating it will fail!")
 
 
 class TaskSetMeta(type):