bpo-36829: Document test.support.catch_unraisable_exception() (GH-13554)

catch_unraisable_exception() now also removes its 'unraisable'
attribute at the context manager exit.

Doc/library/test.rst

@@ -1081,6 +1081,26 @@ The :mod:`test.support` module defines the following functions:
    :exc:`PermissionError` is raised.
 
 
+.. function:: catch_unraisable_exception()
+
+   Context manager catching unraisable exception using
+   :func:`sys.unraisablehook`.
+
+   Usage::
+
+       with support.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)
+
+   .. versionadded:: 3.8
+
+
 .. function:: find_unused_port(family=socket.AF_INET, socktype=socket.SOCK_STREAM)
 
    Returns an unused port that should be suitable for binding.  This is

Lib/test/support/__init__.py

@@ -3043,12 +3043,14 @@ class catch_unraisable_exception:
     Usage:
 
         with support.catch_unraisable_exception() as cm:
+            # code creating an "unraisable exception"
             ...
 
-            # check the expected unraisable exception: use cm.unraisable
+            # check the unraisable exception: use cm.unraisable
             ...
 
-        # cm.unraisable is None here (to break a reference cycle)
+        # cm.unraisable attribute no longer exists at this point
+        # (to break a reference cycle)
     """
 
     def __init__(self):
@@ -3065,5 +3067,5 @@ class catch_unraisable_exception:
 
     def __exit__(self, *exc_info):
         # Clear the unraisable exception to explicitly break a reference cycle
-        self.unraisable = None
+        del self.unraisable
         sys.unraisablehook = self._old_hook

Lib/test/test_io.py

@@ -1098,7 +1098,6 @@ class CommonBufferedTests:
         # Test that the exception state is not modified by a destructor,
         # even if close() fails.
         rawio = self.CloseFailureIO()
-        try:
         with support.catch_unraisable_exception() as cm:
             with self.assertRaises(AttributeError):
                 self.tp(rawio).xyzzy
@@ -1107,9 +1106,6 @@ class CommonBufferedTests:
                 self.assertIsNone(cm.unraisable)
             elif cm.unraisable is not None:
                 self.assertEqual(cm.unraisable.exc_type, OSError)
-        finally:
-            # Explicitly break reference cycle
-            cm = None
 
     def test_repr(self):
         raw = self.MockRawIO()
@@ -2854,7 +2850,6 @@ class TextIOWrapperTest(unittest.TestCase):
         # Test that the exception state is not modified by a destructor,
         # even if close() fails.
         rawio = self.CloseFailureIO()
-        try:
         with support.catch_unraisable_exception() as cm:
             with self.assertRaises(AttributeError):
                 self.TextIOWrapper(rawio).xyzzy
@@ -2863,9 +2858,6 @@ class TextIOWrapperTest(unittest.TestCase):
                 self.assertIsNone(cm.unraisable)
             elif cm.unraisable is not None:
                 self.assertEqual(cm.unraisable.exc_type, OSError)
-        finally:
-            # Explicitly break reference cycle
-            cm = None
 
     # Systematic tests of the text I/O API