diff --git a/pyproject.toml b/pyproject.toml
index 27e3a937d..ee120a181 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,7 +13,7 @@ dependencies = [
   "invoke",
   # lint
   "black",
-  "ruff",
+  "ruff==0.0.278",  # Ruff is moving really fast, so pinning for now.
   "toml",
   "flake8",
   "flake8-pyproject",
diff --git a/src/py/reactpy/reactpy/core/types.py b/src/py/reactpy/reactpy/core/types.py
index 45f300f4f..194706c6e 100644
--- a/src/py/reactpy/reactpy/core/types.py
+++ b/src/py/reactpy/reactpy/core/types.py
@@ -62,21 +62,21 @@ def render(self) -> VdomDict | ComponentType | str | None:
         """Render the component's view model."""
 
 
-_Render = TypeVar("_Render", covariant=True)
-_Event = TypeVar("_Event", contravariant=True)
+_Render_co = TypeVar("_Render_co", covariant=True)
+_Event_contra = TypeVar("_Event_contra", contravariant=True)
 
 
 @runtime_checkable
-class LayoutType(Protocol[_Render, _Event]):
+class LayoutType(Protocol[_Render_co, _Event_contra]):
     """Renders and delivers, updates to views and events to handlers, respectively"""
 
-    async def render(self) -> _Render:
+    async def render(self) -> _Render_co:
         """Render an update to a view"""
 
-    async def deliver(self, event: _Event) -> None:
+    async def deliver(self, event: _Event_contra) -> None:
         """Relay an event to its respective handler"""
 
-    async def __aenter__(self) -> LayoutType[_Render, _Event]:
+    async def __aenter__(self) -> LayoutType[_Render_co, _Event_contra]:
         """Prepare the layout for its first render"""
 
     async def __aexit__(
diff --git a/src/py/reactpy/reactpy/testing/common.py b/src/py/reactpy/reactpy/testing/common.py
index 945c1c31d..6d126fd2e 100644
--- a/src/py/reactpy/reactpy/testing/common.py
+++ b/src/py/reactpy/reactpy/testing/common.py
@@ -25,7 +25,6 @@ def clear_reactpy_web_modules_dir() -> None:
 
 _P = ParamSpec("_P")
 _R = TypeVar("_R")
-_RC = TypeVar("_RC", covariant=True)
 
 
 _DEFAULT_POLL_DELAY = 0.1
diff --git a/src/py/reactpy/reactpy/widgets.py b/src/py/reactpy/reactpy/widgets.py
index cc19be04d..29f941447 100644
--- a/src/py/reactpy/reactpy/widgets.py
+++ b/src/py/reactpy/reactpy/widgets.py
@@ -78,11 +78,11 @@ def sync_inputs(event: dict[str, Any]) -> None:
     return inputs
 
 
-_CastTo = TypeVar("_CastTo", covariant=True)
+_CastTo_co = TypeVar("_CastTo_co", covariant=True)
 
 
-class _CastFunc(Protocol[_CastTo]):
-    def __call__(self, value: str) -> _CastTo:
+class _CastFunc(Protocol[_CastTo_co]):
+    def __call__(self, value: str) -> _CastTo_co:
         ...