Prevent setting method_name in Operation constructor (#41)

The method_name field should be automatically set by the framework to
match the attribute name in service definitions or the method name in
operation handlers. Users should not be able to set it directly.

Changes:
- Add init=False to method_name field in Operation dataclass
- Update framework code to set method_name after Operation construction
- Keep validation to catch manual method_name assignment that doesn't
  match the attribute name
- Remove test case for duplicate method names via constructor (that
  specific scenario is no longer possible)
- Update test expectations to not pass method_name to constructor

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: VegetarianOrc

src/nexusrpc/_service.py

@@ -46,8 +46,7 @@ class MyNexusService:
     """
 
     name: str
-    # TODO(preview): they should not be able to set method_name in constructor
-    method_name: Optional[str] = dataclasses.field(default=None)
+    method_name: Optional[str] = dataclasses.field(default=None, init=False)
     input_type: Optional[type[InputT]] = dataclasses.field(default=None)
     output_type: Optional[type[OutputT]] = dataclasses.field(default=None)
 
@@ -143,10 +142,10 @@ def decorator(cls: type[ServiceT]) -> type[ServiceT]:
             if not hasattr(cls, op_name):
                 op = Operation(
                     name=op_defn.name,
-                    method_name=op_defn.method_name,
                     input_type=op_defn.input_type,
                     output_type=op_defn.output_type,
                 )
+                op.method_name = op_defn.method_name
                 setattr(cls, op_name, op)
 
         return cls
@@ -287,7 +286,6 @@ def _collect_operations(
                     # my_op: Operation[I, O]
                     op = operations[key] = Operation(
                         name=key,
-                        method_name=key,
                         input_type=input_type,
                         output_type=output_type,
                     )
@@ -311,15 +309,14 @@ def _collect_operations(
                 # It looked like
                 # my_op = Operation(...)
                 op = operations[key]
-                if not op.method_name:
-                    op.method_name = key
-                elif op.method_name != key:
-                    raise ValueError(
-                        f"Operation {key} method_name ({op.method_name}) must match attribute name {key}"
-                    )
 
-            if op.method_name is None:
-                op.method_name = key
+            # Validate that if method_name was set (via direct assignment after
+            # construction), it matches the attribute name
+            if op.method_name is not None and op.method_name != key:
+                raise ValueError(
+                    f"Operation {key} method_name ({op.method_name}) must match attribute name {key}"
+                )
+            op.method_name = key
 
         op_defns = {}
         for op in operations.values():

src/nexusrpc/handler/_decorators.py

@@ -174,15 +174,13 @@ def decorator(
                     f"but operation {method.__name__} has {len(type_args)} type parameters: {type_args}"
                 )
 
-        set_operation(
-            method,
-            Operation(
-                name=name or method.__name__,
-                method_name=method.__name__,
-                input_type=input_type,
-                output_type=output_type,
-            ),
+        op: Operation[Any, Any] = Operation(
+            name=name or method.__name__,
+            input_type=input_type,
+            output_type=output_type,
         )
+        op.method_name = method.__name__
+        set_operation(method, op)
         return method
 
     if method is None:
@@ -278,15 +276,13 @@ def _start(ctx: StartOperationContext, input: Any) -> Any:
         )
 
         method_name = get_callable_name(start)
-        set_operation(
-            operation_handler_factory,
-            Operation(
-                name=name or method_name,
-                method_name=method_name,
-                input_type=input_type,
-                output_type=output_type,
-            ),
+        op = Operation(
+            name=name or method_name,
+            input_type=input_type,
+            output_type=output_type,
         )
+        op.method_name = method_name
+        set_operation(operation_handler_factory, op)
 
         set_operation_factory(start, operation_handler_factory)
         return start

tests/handler/test_invalid_usage.py

@@ -124,29 +124,6 @@ def my_op(self, _ctx: StartOperationContext, _input: None) -> None: ...
     error_message = "you have not supplied an executor"
 
 
-class ServiceDefinitionHasDuplicateMethodNames(_TestCase):
-    @staticmethod
-    def build():
-        @nexusrpc.service
-        class SD:
-            my_op: nexusrpc.Operation[None, None] = nexusrpc.Operation(
-                name="my_op",
-                method_name="my_op",
-                input_type=None,
-                output_type=None,
-            )
-            my_op_2: nexusrpc.Operation[None, None] = nexusrpc.Operation(
-                name="my_op_2",
-                method_name="my_op",
-                input_type=None,
-                output_type=None,
-            )
-
-        _ = SD
-
-    error_message = "Operation method name 'my_op' is not unique"
-
-
 class OperationHandlerNoInputOutputTypeAnnotationsWithoutServiceDefinition(_TestCase):
     @staticmethod
     def build():
@@ -189,7 +166,6 @@ async def op(self, ctx, input): ...  # type: ignore[reportMissingParameterType]
         ServiceDefinitionHasExtraOp,
         ServiceHandlerHasExtraOp,
         AsyncioHandlerWithSyncioOperation,
-        ServiceDefinitionHasDuplicateMethodNames,
         OperationHandlerNoInputOutputTypeAnnotationsWithoutServiceDefinition,
         SyncOperationNoTypeAnnotationsWithoutServiceDefinition,
     ],

tests/handler/test_service_handler_decorator_collects_expected_operation_definitions.py

@@ -43,7 +43,6 @@ def operation(self) -> OperationHandler[Input, Output]: ...
     expected_operations = {
         "operation": nexusrpc.Operation(
             name="operation",
-            method_name="operation",
             input_type=Input,
             output_type=Output,
         ),
@@ -59,7 +58,6 @@ def operation(self) -> OperationHandler[Input, Output]: ...
     expected_operations = {
         "operation": nexusrpc.Operation(
             name="operation-name",
-            method_name="operation",
             input_type=Input,
             output_type=Output,
         ),
@@ -77,7 +75,6 @@ def sync_operation_handler(
     expected_operations = {
         "sync_operation_handler": nexusrpc.Operation(
             name="sync_operation_handler",
-            method_name="sync_operation_handler",
             input_type=Input,
             output_type=Output,
         ),
@@ -95,7 +92,6 @@ def sync_operation_handler(
     expected_operations = {
         "sync_operation_handler": nexusrpc.Operation(
             name="sync-operation-name",
-            method_name="sync_operation_handler",
             input_type=Input,
             output_type=Output,
         ),
@@ -115,7 +111,6 @@ def operation(self) -> OperationHandler[Input, Output]: ...
     expected_operations = {
         "operation": nexusrpc.Operation(
             name="operation",
-            method_name="operation",
             input_type=Input,
             output_type=Output,
         ),
@@ -137,7 +132,6 @@ def operation(self) -> OperationHandler[Input, Output]: ...
     expected_operations = {
         "operation": nexusrpc.Operation(
             name="operation-override",
-            method_name="operation",
             input_type=Input,
             output_type=Output,
         ),