1 files changed, 93 insertions, 0 deletions
diff --git a/src/_pytest/unraisableexception.py b/src/_pytest/unraisableexception.py
new file mode 100644
index 000000000..fcb5d8237
--- /dev/null
+++ b/src/_pytest/unraisableexception.py
@@ -0,0 +1,93 @@
+import sys
+import traceback
+import warnings
+from types import TracebackType
+from typing import Any
+from typing import Callable
+from typing import Generator
+from typing import Optional
+from typing import Type
+
+import pytest
+
+
+# Copied from cpython/Lib/test/support/__init__.py, with modifications.
+class catch_unraisable_exception:
+    """Context manager catching unraisable exception using sys.unraisablehook.
+
+    Storing the exception value (cm.unraisable.exc_value) creates a reference
+    cycle. The reference cycle is broken explicitly when the context manager
+    exits.
+
+    Storing the object (cm.unraisable.object) can resurrect it if it is set to
+    an object which is being finalized. Exiting the context manager clears the
+    stored object.
+
+    Usage:
+        with catch_unraisable_exception() as cm:
+            # code creating an "unraisable exception"
+            ...
+            # check the unraisable exception: use cm.unraisable
+            ...
+        # cm.unraisable attribute no longer exists at this point
+        # (to break a reference cycle)
+    """
+
+    def __init__(self) -> None:
+        self.unraisable: Optional["sys.UnraisableHookArgs"] = None
+        self._old_hook: Optional[Callable[["sys.UnraisableHookArgs"], Any]] = None
+
+    def _hook(self, unraisable: "sys.UnraisableHookArgs") -> None:
+        # Storing unraisable.object can resurrect an object which is being
+        # finalized. Storing unraisable.exc_value creates a reference cycle.
+        self.unraisable = unraisable
+
+    def __enter__(self) -> "catch_unraisable_exception":
+        self._old_hook = sys.unraisablehook
+        sys.unraisablehook = self._hook
+        return self
+
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> None:
+        assert self._old_hook is not None
+        sys.unraisablehook = self._old_hook
+        self._old_hook = None
+        del self.unraisable
+
+
+def unraisable_exception_runtest_hook() -> Generator[None, None, None]:
+    with catch_unraisable_exception() as cm:
+        yield
+        if cm.unraisable:
+            if cm.unraisable.err_msg is not None:
+                err_msg = cm.unraisable.err_msg
+            else:
+                err_msg = "Exception ignored in"
+            msg = f"{err_msg}: {cm.unraisable.object!r}\n\n"
+            msg += "".join(
+                traceback.format_exception(
+                    cm.unraisable.exc_type,
+                    cm.unraisable.exc_value,
+                    cm.unraisable.exc_traceback,
+                )
+            )
+            warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
+
+
+@pytest.hookimpl(hookwrapper=True, tryfirst=True)
+def pytest_runtest_setup() -> Generator[None, None, None]:
+    yield from unraisable_exception_runtest_hook()
+
+
+@pytest.hookimpl(hookwrapper=True, tryfirst=True)
+def pytest_runtest_call() -> Generator[None, None, None]:
+    yield from unraisable_exception_runtest_hook()
+
+
+@pytest.hookimpl(hookwrapper=True, tryfirst=True)
+def pytest_runtest_teardown() -> Generator[None, None, None]:
+    yield from unraisable_exception_runtest_hook()