sync primitives

src/textual/app.py

@@ -865,6 +865,17 @@ class App(Generic[ReturnType], DOMNode):
 
         return super().__init_subclass__(*args, **kwargs)
 
+    def _thread_init(self):
+        """Initialize threading primitives for the current thread.
+
+        https://github.com/Textualize/textual/issues/5845
+
+        """
+        self._message_queue
+        self._mounted_event
+        self._exception_event
+        self._thread_id = threading.get_ident()
+
     def _get_dom_base(self) -> DOMNode:
         """When querying from the app, we want to query the default screen."""
         return self.default_screen
@@ -2094,8 +2105,9 @@ class App(Generic[ReturnType], DOMNode):
                     run_auto_pilot(auto_pilot, pilot), name=repr(pilot)
                 )
 
+        self._thread_init()
+
         app._loop = asyncio.get_running_loop()
-        app._thread_id = threading.get_ident()
         with app._context():
             try:
                 await app._process_messages(
@@ -3120,6 +3132,9 @@ class App(Generic[ReturnType], DOMNode):
         terminal_size: tuple[int, int] | None = None,
         message_hook: Callable[[Message], None] | None = None,
     ) -> None:
+
+        self._thread_init()
+
         async def app_prelude() -> bool:
             """Work required before running the app.
 

src/textual/message_pump.py

@@ -11,7 +11,6 @@ A `MessagePump` is a base class for any object which processes messages, which i
 from __future__ import annotations
 
 import asyncio
-import sys
 import threading
 from asyncio import CancelledError, QueueEmpty, Task, create_task
 from contextlib import contextmanager
@@ -23,12 +22,10 @@ from typing import (
     Awaitable,
     Callable,
     Generator,
-    Generic,
     Iterable,
     Type,
     TypeVar,
     cast,
-    overload,
 )
 from weakref import WeakSet
 
@@ -163,6 +160,15 @@ class MessagePump(metaclass=_MessagePumpMeta):
             prevent_message_types_stack.set(stack)
         return stack
 
+    def _thread_init(self):
+        """Initialize threading primitives for the current thread.
+
+        Require for Python3.8 https://github.com/Textualize/textual/issues/5845
+
+        """
+        self._message_queue
+        self._mounted_event
+
     def _get_prevented_messages(self) -> set[type[Message]]:
         """A set of all the prevented message types."""
         return self._prevent_message_types_stack[-1]
@@ -506,6 +512,8 @@ class MessagePump(metaclass=_MessagePumpMeta):
 
     def _start_messages(self) -> None:
         """Start messages task."""
+        self._thread_init()
+
         if self.app._running:
             self._task = create_task(
                 self._process_messages(), name=f"message pump {self}"

src/textual/timer.py

@@ -152,6 +152,7 @@ class Timer:
         count = 0
         _repeat = self._repeat
         _interval = self._interval
+        self._active  # Force instantiation in same thread
         await self._active.wait()
         start = _time.get_time()