Address picnixz review: discard deferred error on any exceptional exit

Author: arpittkhandelwal

Lib/_pydecimal.py

@@ -4165,16 +4165,18 @@ def _raise_error(self, condition, explanation = None, *args):
     def _defer_traps(self):
         """Defer trapped signals until the end of the with block.
 
-        _raise_error sets the flag of every signal in the block and
+        _raise_error() sets the flag of every signal in the block and
         raises the first trapped exception on exit, so that a
         multi-signal operation sets the same flags as _decimal.
         """
         self._deferred_traps += 1
         try:
             yield
         except BaseException:
-            if self._deferred_traps == 1:
-                self._deferred_error = None
+            # The block is aborted, so the deferred error is
+            # discarded: the propagating exception wins, and the
+            # context does not retain an exception object.
+            self._deferred_error = None
             raise
         finally:
             self._deferred_traps -= 1

Lib/test/test_decimal.py

@@ -4311,6 +4311,32 @@ def test_deferred_traps_attributes(self):
         self.assertEqual(context._deferred_traps, 0)
         self.assertIsNone(context._deferred_error)
 
+    def test_deferred_traps_exception_discards_error(self):
+        # If an exception propagates through a _defer_traps block, the
+        # deferred error is discarded at every nesting level and the
+        # state is reset, so the propagating exception wins and the
+        # context stays usable.
+        Context = self.decimal.Context
+        Decimal = self.decimal.Decimal
+        Overflow = self.decimal.Overflow
+        context = Context(prec=9, Emin=-999, Emax=999, traps=[Overflow])
+        with self.assertRaises(RuntimeError):
+            with context._defer_traps():
+                with context._defer_traps():
+                    context._raise_error(Overflow, 'test', 1)
+                    raise RuntimeError('boom')
+        self.assertEqual(context._deferred_traps, 0)
+        self.assertIsNone(context._deferred_error)
+
+        # The flag was still set, and a later trapped operation
+        # raises its own error rather than the discarded one.
+        self.assertTrue(context.flags[Overflow])
+        context.clear_flags()
+        with self.assertRaises(Overflow):
+            context.multiply(Decimal('1E+800'), Decimal('1E+800'))
+        self.assertEqual(context._deferred_traps, 0)
+        self.assertIsNone(context._deferred_error)
+
 class SpecialContexts:
     """Test the context templates."""