Add SimpleHandler to make minimal Attributes functional

This allows creating Attributes without a Handler for parameters that
are only used for internal controller logic and not sent directly to a
device.

src/fastcs/attributes.py

@@ -41,6 +41,19 @@ class Handler(Sender, Updater, Protocol):
     pass
 
 
+class SimpleHandler(Handler):
+    """Handler for internal parameters"""
+
+    async def put(self, controller: Any, attr: AttrW, value: Any):
+        await attr.update_display_without_process(value)
+
+        if isinstance(attr, AttrRW):
+            await attr.set(value)
+
+    async def update(self, controller: Any, attr: AttrR):
+        raise RuntimeError("SimpleHandler cannot update")
+
+
 class Attribute(Generic[T]):
     """Base FastCS attribute.
 
@@ -171,7 +184,11 @@ def __init__(
         )
         self._process_callback: AttrCallback[T] | None = None
         self._write_display_callback: AttrCallback[T] | None = None
-        self._sender = handler
+
+        if handler is not None:
+            self._sender = handler
+        else:
+            self._sender = SimpleHandler()
 
     async def process(self, value: T) -> None:
         await self.process_without_display_update(value)
@@ -195,7 +212,7 @@ def set_write_display_callback(self, callback: AttrCallback[T] | None) -> None:
         self._write_display_callback = callback
 
     @property
-    def sender(self) -> Sender | None:
+    def sender(self) -> Sender:
         return self._sender
 
 

tests/test_attribute.py

@@ -1,8 +1,9 @@
 from functools import partial
 
 import pytest
+from pytest_mock import MockerFixture
 
-from fastcs.attributes import AttrR, AttrRW
+from fastcs.attributes import AttrR, AttrRW, AttrW
 from fastcs.datatypes import Int, String
 
 
@@ -31,3 +32,28 @@ async def device_add():
     await attr_rw.process(2)
     assert device["number"] == 2
     assert ui["number"] == 2
+
+
+@pytest.mark.asyncio
+async def test_simple_handler_w(mocker: MockerFixture):
+    attr = AttrW(Int())
+    update_display_mock = mocker.patch.object(attr, "update_display_without_process")
+
+    # This is called by the transport when it receives a put
+    await attr.sender.put(mocker.ANY, attr, 1)
+
+    # The callback to update the transport display should be called
+    update_display_mock.assert_called_once_with(1)
+
+
+@pytest.mark.asyncio
+async def test_simple_handler_rw(mocker: MockerFixture):
+    attr = AttrRW(Int())
+    update_display_mock = mocker.patch.object(attr, "update_display_without_process")
+    set_mock = mocker.patch.object(attr, "set")
+
+    await attr.sender.put(mocker.ANY, attr, 1)
+
+    update_display_mock.assert_called_once_with(1)
+    # The Sender of the attribute should just set the value on the attribute
+    set_mock.assert_awaited_once_with(1)