Warn when activity method is registered without an instance (#1918)

Co-authored-by: Tim Conley <tconley1428@gmail.com>

Author: vjymisal0

CHANGELOG.md

@@ -20,6 +20,8 @@ to include examples, links to docs, or any other relevant information.
 
 ### Added
 
+- Worker now warns when an activity method is registered from the class
+  instead of from an instance (i.e. its first parameter is an unbound `self`).
 - `workflow.new_random()` accepts an optional `name` that is mixed into the seed, so differently
   named generators, and `workflow.random()`, produce different sequences.
 

temporalio/worker/_activity.py

@@ -95,6 +95,17 @@ def __init__(
                     f"Activity named {defn.name} is a class instead of an instance"
                 )
 
+            # A plain function whose first parameter is "self" is almost always
+            # a method referenced from the class instead of from an instance
+            if inspect.isfunction(activity):
+                params = list(inspect.signature(activity).parameters)
+                if params and params[0] == "self":
+                    warnings.warn(
+                        f"Activity named {defn.name} has a first parameter named self "
+                        "but is not bound to an instance, did you mean to register it "
+                        "from an instance of the class?"
+                    )
+
             # Some extra requirements for sync functions
             if not defn.is_async:
                 if not activity_executor:

tests/worker/test_workflow.py

@@ -4188,6 +4188,12 @@ async def test_workflow_activity_method(client: Client):
         assert result == MyDataClass(field1="in worker, workflow param, in workflow")
 
 
+async def test_workflow_activity_method_unbound_register(client: Client):
+    # Registering the method from the class instead of from an instance
+    with pytest.warns(UserWarning, match="is not bound to an instance"):
+        new_worker(client, ActivityMethodWorkflow, activities=[MethodActivity.add])
+
+
 @workflow.defn
 class WaitConditionTimeoutWorkflow:
     def __init__(self) -> None: