fix: Reject duplicate handler names at registration

python/restate/object.py

@@ -175,7 +175,7 @@ def handler(
             Callable: The decorated function.
 
         Raises:
-            ValueError: If the handler name is not provided.
+            ValueError: If the handler name is not provided, or a handler with the same name is already registered.
 
         Example:
             @service.handler()
@@ -215,6 +215,8 @@ def wrapped(*args, **kwargs):
                 invocation_retry_policy,
                 combined_context_managers,
             )
+            if handler.name in self.handlers:
+                raise ValueError(f"Handler {handler.name} already exists in {self.name}")
             self.handlers[handler.name] = handler
             return wrapped
 

python/restate/service.py

@@ -163,7 +163,7 @@ def handler(
             Callable: The decorated function.
 
         Raises:
-            ValueError: If the handler name is not provided.
+            ValueError: If the handler name is not provided, or a handler with the same name is already registered.
 
         Example:
             @service.handler()
@@ -206,6 +206,8 @@ def wrapped(*args, **kwargs):
                 invocation_retry_policy,
                 combined_context_managers,
             )
+            if handler.name in self.handlers:
+                raise ValueError(f"Handler {handler.name} already exists in {self.name}")
             self.handlers[handler.name] = handler
             return wrapped
 

python/restate/workflow.py

@@ -342,7 +342,7 @@ def _add_handler(
             Callable: The decorated function.
 
         Raises:
-            ValueError: If the handler name is not provided.
+            ValueError: If the handler name is not provided, or a handler with the same name is already registered.
 
         Example:
             @service.handler()
@@ -383,6 +383,8 @@ def wrapped(*args, **kwargs):
                 invocation_retry_policy=invocation_retry_policy,
                 context_managers=combined_context_managers,
             )
+            if handler.name in self.handlers:
+                raise ValueError(f"Handler {handler.name} already exists in {self.name}")
             self.handlers[handler.name] = handler
             return wrapped
 

tests/duplicate_handlers.py

@@ -0,0 +1,54 @@
+#
+#  Copyright (c) 2023-2026 - Restate Software, Inc., Restate GmbH
+#
+#  This file is part of the Restate SDK for Python,
+#  which is released under the MIT license.
+#
+#  You can find a copy of the license in file LICENSE in the root
+#  directory of this repository or package, or at
+#  https://github.com/restatedev/sdk-typescript/blob/main/LICENSE
+#
+import pytest
+import restate
+
+
+def test_service_rejects_duplicate_handler_name():
+    service = restate.Service("Greeter")
+
+    @service.handler("greet")
+    async def first(ctx: restate.Context) -> str:
+        return "first"
+
+    with pytest.raises(ValueError, match="Handler greet already exists in Greeter"):
+
+        @service.handler("greet")
+        async def second(ctx: restate.Context) -> str:
+            return "second"
+
+
+def test_virtual_object_rejects_duplicate_handler_name():
+    obj = restate.VirtualObject("Counter")
+
+    @obj.handler("add")
+    async def first(ctx: restate.ObjectContext) -> int:
+        return 1
+
+    with pytest.raises(ValueError, match="Handler add already exists in Counter"):
+
+        @obj.handler("add")
+        async def second(ctx: restate.ObjectContext) -> int:
+            return 2
+
+
+def test_workflow_rejects_duplicate_handler_name():
+    workflow = restate.Workflow("Signup")
+
+    @workflow.main("run")
+    async def run(ctx: restate.WorkflowContext) -> str:
+        return "run"
+
+    with pytest.raises(ValueError, match="Handler run already exists in Signup"):
+
+        @workflow.handler("run")
+        async def other(ctx: restate.WorkflowSharedContext) -> str:
+            return "other"