Allow early return from effects (#121)

counterweight/_utils.py

@@ -81,7 +81,14 @@ async def forever() -> None:
 
 async def cancel(task: Task[T]) -> None:
     # Based on https://discuss.python.org/t/asyncio-cancel-a-cancellation-utility-as-a-coroutine-this-time-with-feeling/26304/2
+    if task.done():
+        # If the task has already completed, there's nothing to cancel.
+        # This can happen if, for example, an effect aborts itself by returning,
+        # and then we try to cancel it when reconciling effects.
+        return
+
     task.cancel()
+
     try:
         await task
     except CancelledError:

docs/reference/changelog.md

@@ -2,8 +2,22 @@
 
 ## Next
 
+## `0.0.9`
+
+Released `2024-02-27`
+
+### Fixed
+
+- [#121](https://github.com/JoshKarpel/counterweight/pull/121)
+  A change in effect reconciliation introduced in `0.0.8` caused a regression in the behavior of `use_effect`,
+  where if the `setup` function `return`ed (i.e., stopped itself),
+  Counterweight would crash when trying to cancel the effect.
+  This now works again.
+
 ## `0.0.8`
 
+Released `2024-02-17`
+
 ### Changed
 
 - [#110](https://github.com/JoshKarpel/counterweight/pull/110)

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "poetry.core.masonry.api"
 
 [tool.poetry]
 name = "counterweight"
-version = "0.0.8"
+version = "0.0.9"
 description = "An experimental TUI framework for Python, inspired by React and Tailwind"
 readme="README.md"
 homepage="https://github.com/JoshKarpel/counterweight"

tests/utils/test_cancel.py

@@ -50,3 +50,14 @@ async def t() -> None:
 
     with pytest.raises(RuntimeError):
         await cancel(task)
+
+
+async def test_cancel_with_task_that_has_already_finished() -> None:
+    async def t() -> None:
+        return
+
+    task = create_task(t())
+
+    await task  # run the task to completion
+
+    await cancel(task)