[Python compatibility] Make the code work with Python 3.7+

src/textual/_compositor.py

@@ -448,7 +448,7 @@ class Compositor:
         segment_lines: list[list[Segment]] = [
             sum(
                 [line for line in bucket.values() if line is not None],
-                start=[],
+                [],
             )
             for bucket in chops
         ]

src/textual/_timer.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import weakref
 from asyncio import (
     get_event_loop,
@@ -84,7 +85,7 @@ class Timer:
         Returns:
             Task: A Task instance for the timer.
         """
-        self._task = get_event_loop().create_task(self._run())
+        self._task = asyncio.create_task(self._run())
         return self._task
 
     async def stop(self) -> None:
@@ -114,7 +115,12 @@ class Timer:
                     continue
                 wait_time = max(0, next_timer - monotonic())
                 if wait_time:
-                    await sleep(wait_time)
+                    try:
+                        await sleep(wait_time)
+                    except asyncio.CancelledError:
+                        # Likely our program terminating: this is fine, we just have to
+                        # shut down out asyncio Task properly:
+                        await self.stop()
                 event = events.Timer(
                     self.sender,
                     timer=self,

src/textual/app.py

@@ -54,6 +54,9 @@ WINDOWS = PLATFORM == "Windows"
 # asyncio will warn against resources not being cleared
 warnings.simplefilter("always", ResourceWarning)
 
+# `asyncio.get_event_loop()` is deprecated since Python 3.10:
+_ASYNCIO_GET_EVENT_LOOP_IS_DEPRECATED = sys.version_info >= (3, 10, 0)
+
 LayoutDefinition = "dict[str, Any]"
 
 
@@ -109,6 +112,8 @@ class App(Generic[ReturnType], DOMNode):
             css (str | None, optional): CSS code to parse, or ``None`` for no literal CSS. Defaults to None.
             watch_css (bool, optional): Watch CSS for changes. Defaults to True.
         """
+        App._init_uvloop()
+
         self.console = Console(
             file=sys.__stdout__, markup=False, highlight=False, emoji=False
         )
@@ -319,28 +324,33 @@ class App(Generic[ReturnType], DOMNode):
             keys, action, description, show=show, key_display=key_display
         )
 
-    def run(self, loop: AbstractEventLoop | None = None) -> ReturnType | None:
+    def run(self) -> ReturnType | None:
         async def run_app() -> None:
             await self.process_messages()
 
-        if loop:
-            asyncio.set_event_loop(loop)
-        else:
-            try:
-                import uvloop
-            except ImportError:
-                pass
-            else:
-                asyncio.set_event_loop(uvloop.new_event_loop())
-
-        event_loop = asyncio.get_event_loop()
-        try:
+        if _ASYNCIO_GET_EVENT_LOOP_IS_DEPRECATED:
+            # N.B. This doesn't work with Python<3.10, as we end up with 2 event loops:
             asyncio.run(run_app())
-        finally:
-            event_loop.close()
+        else:
+            # However, this works with Python<3.10:
+            event_loop = asyncio.get_event_loop()
+            event_loop.run_until_complete(run_app())
 
         return self._return_value
 
+    @classmethod
+    def _init_uvloop(cls) -> None:
+        if hasattr(cls, "__uvloop_installed"):
+            return
+        cls.__uvloop_installed = False
+        try:
+            import uvloop
+        except ImportError:
+            pass
+        else:
+            uvloop.install()
+            cls.__uvloop_installed = True
+
     async def _on_css_change(self) -> None:
         """Called when the CSS changes (if watch_css is True)."""
         if self.css_file is not None:
@@ -508,6 +518,7 @@ class App(Generic[ReturnType], DOMNode):
         active_app.set(self)
         log("---")
         log(f"driver={self.driver_class}")
+        log(f"uvloop installed: {self.__class__.__uvloop_installed!r}")
 
         if self.devtools_enabled:
             try: