Ensure backend creates sender callbacks with the correct controller

src/fastcs/backend.py

@@ -30,7 +30,10 @@ def __init__(
     def _link_process_tasks(self):
         for controller_api in self.controller_api.walk_api():
             _link_put_tasks(controller_api)
-            _link_attribute_sender_class(controller_api, self._controller)
+            _link_attribute_sender_class(
+                controller_api,
+                self._controller.get_controller_by_path(controller_api.path),
+            )
 
     def __del__(self):
         self._stop_scan_tasks()
@@ -74,7 +77,7 @@ def _link_put_tasks(controller_api: ControllerAPI) -> None:
 
 
 def _link_attribute_sender_class(
-    controller_api: ControllerAPI, controller: Controller
+    controller_api: ControllerAPI, controller: BaseController
 ) -> None:
     for attr_name, attribute in controller_api.attributes.items():
         match attribute:

src/fastcs/controller.py

@@ -4,6 +4,7 @@
 from typing import get_type_hints
 
 from fastcs.attributes import Attribute
+from fastcs.exceptions import FastCSException
 
 
 class BaseController:
@@ -101,6 +102,19 @@ def register_sub_controller(self, name: str, sub_controller: SubController):
     def get_sub_controllers(self) -> dict[str, SubController]:
         return self.__sub_controller_tree
 
+    def __getitem__(self, key: str) -> SubController:
+        if key not in self.__sub_controller_tree:
+            raise FastCSException(f"Controller {self} has no sub controller '{key}'")
+
+        return self.__sub_controller_tree[key]
+
+    def get_controller_by_path(self, path: list[str]) -> BaseController:
+        if path:
+            sub_controller, remaining_path = self[path[0]], path[1:]
+            return sub_controller.get_controller_by_path(remaining_path)
+
+        return self
+
 
 class Controller(BaseController):
     """Top-level controller for a device.

tests/test_backend.py

@@ -1,10 +1,13 @@
 import asyncio
+from unittest.mock import call
+
+from pytest_mock import MockerFixture
 
 from fastcs.attributes import AttrRW
 from fastcs.backend import Backend, build_controller_api
-from fastcs.controller import Controller
+from fastcs.controller import Controller, SubController
 from fastcs.cs_methods import Command
-from fastcs.datatypes import Int
+from fastcs.datatypes import Float, Int
 from fastcs.wrappers import command, scan
 
 
@@ -36,6 +39,29 @@ async def test_wrapper():
     loop.run_until_complete(test_wrapper())
 
 
+def test_backend_link_attribute_sender(mocker: MockerFixture):
+    class MyController(Controller):
+        attr = AttrRW(Float())
+
+    class MySubController(SubController):
+        sub_attr = AttrRW(Int())
+
+    controller = MyController()
+    sub_controller = MySubController()
+    controller.register_sub_controller("sub", sub_controller)
+
+    loop = asyncio.get_event_loop()
+    create_sender_mock = mocker.patch("fastcs.backend._create_sender_callback")
+    Backend(controller, loop)
+
+    create_sender_mock.assert_has_calls(
+        [
+            call(controller.attr, controller),
+            call(sub_controller.sub_attr, sub_controller),
+        ]
+    )
+
+
 def test_controller_api():
     class MyTestController(Controller):
         attr1: AttrRW[int] = AttrRW(Int())

tests/test_controller.py

@@ -3,6 +3,7 @@
 from fastcs.attributes import AttrR
 from fastcs.controller import Controller, SubController
 from fastcs.datatypes import Int
+from fastcs.exceptions import FastCSException
 
 
 def test_controller_nesting():
@@ -16,7 +17,9 @@ def test_controller_nesting():
     assert sub_controller.path == ["a"]
     assert sub_sub_controller.path == ["a", "b"]
     assert controller.get_sub_controllers() == {"a": sub_controller}
+    assert controller["a"] == sub_controller
     assert sub_controller.get_sub_controllers() == {"b": sub_sub_controller}
+    assert controller["a"]["b"] == sub_sub_controller
 
     with pytest.raises(
         ValueError, match=r"Controller .* already has a SubController registered as .*"
@@ -112,3 +115,12 @@ class FailingController(SomeController):
         ),
     ):
         FailingController(SomeSubController())
+
+
+def test_controller_getitem(controller):
+    assert controller["SubController01"] == controller.get_sub_controllers().get(
+        "SubController01"
+    )
+
+    with pytest.raises(FastCSException, match=r"Controller .* has no sub controller.*"):
+        controller["DoesNotExist"]