bpo-42482: remove reference to exc_traceback from TracebackException (GH-23531)

(cherry picked from commit 427613f)

Co-authored-by: Irit Katriel <[email protected]>

Author: miss-islington

Lib/test/test_traceback.py

@@ -1083,6 +1083,18 @@ def test_context(self):
         self.assertEqual(exc_info[0], exc.exc_type)
         self.assertEqual(str(exc_info[1]), str(exc))
 
+    def test_no_refs_to_exception_and_traceback_objects(self):
+        try:
+            1/0
+        except Exception:
+            exc_info = sys.exc_info()
+
+        refcnt1 = sys.getrefcount(exc_info[1])
+        refcnt2 = sys.getrefcount(exc_info[2])
+        exc = traceback.TracebackException(*exc_info)
+        self.assertEqual(sys.getrefcount(exc_info[1]), refcnt1)
+        self.assertEqual(sys.getrefcount(exc_info[2]), refcnt2)
+
     def test_comparison_basic(self):
         try:
             1/0
@@ -1133,6 +1145,16 @@ def raise_with_locals():
         exc7 = traceback.TracebackException(*exc_info, limit=-2, capture_locals=True)
         self.assertNotEqual(exc6, exc7)
 
+    def test_comparison_equivalent_exceptions_are_equal(self):
+        excs = []
+        for _ in range(2):
+            try:
+                1/0
+            except:
+                excs.append(traceback.TracebackException(*sys.exc_info()))
+        self.assertEqual(excs[0], excs[1])
+        self.assertEqual(list(excs[0].format()), list(excs[1].format()))
+
     def test_unhashable(self):
         class UnhashableException(Exception):
             def __eq__(self, other):

Lib/traceback.py

@@ -500,7 +500,6 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
                 _seen=_seen)
         else:
             context = None
-        self.exc_traceback = exc_traceback
         self.__cause__ = cause
         self.__context__ = context
         self.__suppress_context__ = \
@@ -608,7 +607,7 @@ def format(self, *, chain=True):
                 not self.__suppress_context__):
                 yield from self.__context__.format(chain=chain)
                 yield _context_message
-        if self.exc_traceback is not None:
+        if self.stack:
             yield 'Traceback (most recent call last):\n'
-        yield from self.stack.format()
+            yield from self.stack.format()
         yield from self.format_exception_only()

Misc/NEWS.d/next/Library/2020-11-27-16-46-58.bpo-42482.EJC3sd.rst

@@ -0,0 +1 @@
+:class:`~traceback.TracebackException` no longer holds a reference to the exception's traceback object. Consequently, instances of TracebackException for equivalent but non-equal exceptions now compare as equal.