Define trio.Event.__bool__() to reduce bugs

newsfragments/3322.deprecated.txt

@@ -0,0 +1 @@
+Implement ``bool(trio.Event)`` and have it raise a `DeprecationWarning` and tell users to use `trio.Event.is_set` instead. This is an alternative to ``mypy --enable-error-code=truthy-bool`` for users who don't use type checking.

src/trio/_sync.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import math
-from typing import TYPE_CHECKING, Protocol
+from typing import TYPE_CHECKING, Literal, Protocol, TypeVar
 
 import attrs
 
@@ -16,13 +16,31 @@
     enable_ki_protection,
     remove_parking_lot_breaker,
 )
+from ._deprecate import warn_deprecated
 from ._util import final
 
 if TYPE_CHECKING:
+    from collections.abc import Callable
     from types import TracebackType
 
+    from typing_extensions import deprecated
+
     from ._core import Task
     from ._core._parking_lot import ParkingLotStatistics
+else:
+    T = TypeVar("T")
+
+    def deprecated(
+        message: str,
+        /,
+        *,
+        category: type[Warning] | None = DeprecationWarning,
+        stacklevel: int = 1,
+    ) -> Callable[[T], T]:
+        def wrapper(f: T) -> T:
+            return f
+
+        return wrapper
 
 
 @attrs.frozen
@@ -112,6 +130,20 @@ def statistics(self) -> EventStatistics:
         """
         return EventStatistics(tasks_waiting=len(self._tasks))
 
+    @deprecated(
+        "trio.Event.__bool__ is deprecated since Trio 0.31.0; use trio.Event.is_set instead (https://github.com/python-trio/trio/issues/3238)",
+        stacklevel=2,
+    )
+    def __bool__(self) -> Literal[True]:
+        """Return True and raise warning."""
+        warn_deprecated(
+            self.__bool__,
+            "0.31.0",
+            issue=3238,
+            instead=self.is_set,
+        )
+        return True
+
 
 class _HasAcquireRelease(Protocol):
     """Only classes with acquire() and release() can use the mixin's implementations."""

src/trio/_tests/test_sync.py

@@ -23,6 +23,12 @@ async def test_Event() -> None:
     assert not e.is_set()
     assert e.statistics().tasks_waiting == 0
 
+    with pytest.warns(
+        DeprecationWarning,
+        match=r"trio\.Event\.__bool__ is deprecated since Trio 0\.31\.0; use trio\.Event\.is_set instead \(https://github.com/python-trio/trio/issues/3238\)",
+    ):
+        e.__bool__()
+
     e.set()
     assert e.is_set()
     with assert_checkpoints():

test-requirements.txt

@@ -187,7 +187,7 @@ types-pyyaml==6.0.12.20250822
     # via -r test-requirements.in
 types-setuptools==80.9.0.20250822
     # via types-cffi
-typing-extensions==4.14.1
+typing-extensions==4.15.0
     # via
     #   -r test-requirements.in
     #   astroid