Add unraisableexception and threadexception plugins

changelog/5299.feature.rst

@@ -0,0 +1,2 @@
+pytest now warns about unraisable exceptions and unhandled thread exceptions that occur in tests on Python>=3.8.
+See :ref:`unraisable` for more information.

doc/en/reference.rst

@@ -1090,6 +1090,12 @@ Custom warnings generated in some situations such as improper usage or deprecate
 .. autoclass:: pytest.PytestUnknownMarkWarning
    :show-inheritance:
 
+.. autoclass:: pytest.PytestUnraisableExceptionWarning
+   :show-inheritance:
+
+.. autoclass:: pytest.PytestUnhandledThreadExceptionWarning
+   :show-inheritance:
+
 
 Consult the :ref:`internal-warnings` section in the documentation for more information.
 

doc/en/usage.rst

@@ -470,6 +470,38 @@ seconds to finish (not available on Windows).
       the command-line using ``-o faulthandler_timeout=X``.
 
 
+.. _unraisable:
+
+Warning about unraisable exceptions and unhandled thread exceptions
+-------------------------------------------------------------------
+
+.. versionadded:: 6.2
+
+.. note::
+
+    These features only work on Python>=3.8.
+
+Unhandled exceptions are exceptions that are raised in a situation in which
+they cannot propagate to a caller. The most common case is an exception raised
+in a :meth:`__del__ <object.__del__>` implementation.
+
+Unhandled thread exceptions are exceptions raised in a :class:`~threading.Thread`
+but not handled, causing the thread to terminate uncleanly.
+
+Both types of exceptions are normally considered bugs, but may go unnoticed
+because they don't cause the program itself to crash. Pytest detects these
+conditions and issues a warning that is visible in the test run summary.
+
+The plugins are automatically enabled for pytest runs, unless the
+``-p no:unraisableexception`` (for unraisable exceptions) and
+``-p no:threadexception`` (for thread exceptions) options are given on the
+command-line.
+
+The warnings may be silenced selectivly using the :ref:`pytest.mark.filterwarnings ref`
+mark. The warning categories are :class:`pytest.PytestUnraisableExceptionWarning` and
+:class:`pytest.PytestUnhandledThreadExceptionWarning`.
+
+
 Creating JUnitXML format files
 ----------------------------------------------------
 

src/_pytest/config/__init__.py

@@ -251,6 +251,7 @@ def directory_arg(path: str, optname: str) -> str:
     "warnings",
     "logging",
     "reports",
+    *(["unraisableexception", "threadexception"] if sys.version_info >= (3, 8) else []),
     "faulthandler",
 )
 

src/_pytest/pytester.py

@@ -1349,7 +1349,7 @@ def run(
                 stderr=f2,
                 close_fds=(sys.platform != "win32"),
             )
-            if isinstance(stdin, bytes):
+            if popen.stdin is not None:
                 popen.stdin.close()
 
             def handle_timeout() -> None:

src/_pytest/threadexception.py

@@ -0,0 +1,90 @@
+import threading
+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/threading_helper.py, with modifications.
+class catch_threading_exception:
+    """Context manager catching threading.Thread exception using
+    threading.excepthook.
+
+    Storing exc_value using a custom hook can create a reference cycle. The
+    reference cycle is broken explicitly when the context manager exits.
+
+    Storing thread using a custom hook can resurrect it if it is set to an
+    object which is being finalized. Exiting the context manager clears the
+    stored object.
+
+    Usage:
+        with threading_helper.catch_threading_exception() as cm:
+            # code spawning a thread which raises an exception
+            ...
+            # check the thread exception: use cm.args
+            ...
+        # cm.args attribute no longer exists at this point
+        # (to break a reference cycle)
+    """
+
+    def __init__(self) -> None:
+        # See https://github.com/python/typeshed/issues/4767 regarding the underscore.
+        self.args: Optional["threading._ExceptHookArgs"] = None
+        self._old_hook: Optional[Callable[["threading._ExceptHookArgs"], Any]] = None
+
+    def _hook(self, args: "threading._ExceptHookArgs") -> None:
+        self.args = args
+
+    def __enter__(self) -> "catch_threading_exception":
+        self._old_hook = threading.excepthook
+        threading.excepthook = 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
+        threading.excepthook = self._old_hook
+        self._old_hook = None
+        del self.args
+
+
+def thread_exception_runtest_hook() -> Generator[None, None, None]:
+    with catch_threading_exception() as cm:
+        yield
+        if cm.args:
+            if cm.args.thread is not None:
+                thread_name = cm.args.thread.name
+            else:
+                thread_name = "<unknown>"
+            msg = f"Exception in thread {thread_name}\n\n"
+            msg += "".join(
+                traceback.format_exception(
+                    cm.args.exc_type, cm.args.exc_value, cm.args.exc_traceback,
+                )
+            )
+            warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))
+
+
+@pytest.hookimpl(hookwrapper=True, trylast=True)
+def pytest_runtest_setup() -> Generator[None, None, None]:
+    yield from thread_exception_runtest_hook()
+
+
+@pytest.hookimpl(hookwrapper=True, tryfirst=True)
+def pytest_runtest_call() -> Generator[None, None, None]:
+    yield from thread_exception_runtest_hook()
+
+
+@pytest.hookimpl(hookwrapper=True, tryfirst=True)
+def pytest_runtest_teardown() -> Generator[None, None, None]:
+    yield from thread_exception_runtest_hook()

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()

src/_pytest/warning_types.py

@@ -90,6 +90,28 @@ class PytestUnknownMarkWarning(PytestWarning):
     __module__ = "pytest"
 
 
+@final
+class PytestUnraisableExceptionWarning(PytestWarning):
+    """An unraisable exception was reported.
+
+    Unraisable exceptions are exceptions raised in :meth:`__del__ <object.__del__>`
+    implementations and similar situations when the exception cannot be raised
+    as normal.
+    """
+
+    __module__ = "pytest"
+
+
+@final
+class PytestUnhandledThreadExceptionWarning(PytestWarning):
+    """An unhandled exception occurred in a :class:`~threading.Thread`.
+
+    Such exceptions don't propagate normally.
+    """
+
+    __module__ = "pytest"
+
+
 _W = TypeVar("_W", bound=PytestWarning)
 
 

src/pytest/__init__.py

@@ -44,7 +44,9 @@
 from _pytest.warning_types import PytestDeprecationWarning
 from _pytest.warning_types import PytestExperimentalApiWarning
 from _pytest.warning_types import PytestUnhandledCoroutineWarning
+from _pytest.warning_types import PytestUnhandledThreadExceptionWarning
 from _pytest.warning_types import PytestUnknownMarkWarning
+from _pytest.warning_types import PytestUnraisableExceptionWarning
 from _pytest.warning_types import PytestWarning
 
 set_trace = __pytestPDB.set_trace
@@ -85,7 +87,9 @@
     "PytestDeprecationWarning",
     "PytestExperimentalApiWarning",
     "PytestUnhandledCoroutineWarning",
+    "PytestUnhandledThreadExceptionWarning",
     "PytestUnknownMarkWarning",
+    "PytestUnraisableExceptionWarning",
     "PytestWarning",
     "raises",
     "register_assert_rewrite",

testing/acceptance_test.py

@@ -1288,3 +1288,6 @@ def test_no_brokenpipeerror_message(pytester: Pytester) -> None:
     ret = popen.wait()
     assert popen.stderr.read() == b""
     assert ret == 1
+
+    # Cleanup.
+    popen.stderr.close()