bpo-42247: fix unittest.assertRaises bug where traceback entries were…

… dropped from the chained exception
python · iritkatriel · Mar 8, 2022 · Dec 8, 2020 · Sep 5, 2021 · Sep 5, 2021
commit f642752b725ea1867f22be6bfa2e3f951070d707
diff --git a/Lib/unittest/result.py b/Lib/unittest/result.py
@@ -179,12 +179,10 @@ def _exc_info_to_string(self, err, test):
 
         if exctype is test.failureException:
             # Skip assert*() traceback levels
-            length = self._count_relevant_tb_levels(tb)
-        else:
-            length = None
+            self._remove_unittest_tb_frames(tb)
         tb_e = traceback.TracebackException(
             exctype, value, tb,
-            limit=length, capture_locals=self.tb_locals, compact=True)
+            capture_locals=self.tb_locals, compact=True)
         msgLines = list(tb_e.format())
 
         if self.buffer:
@@ -204,12 +202,20 @@ def _exc_info_to_string(self, err, test):
     def _is_relevant_tb_level(self, tb):
         return '__unittest' in tb.tb_frame.f_globals
 
-    def _count_relevant_tb_levels(self, tb):
-        length = 0
+    def _remove_unittest_tb_frames(self, tb):
+        '''Truncates usercode tb at the first unittest frame.
+
+        If the first frame of the traceback is in user code,
+        the prefix up to the first unittest frame is returned.
+        If the first frame is already in the unittest module,
+        the traceback is not modified.
+        '''
+        prev = None
         while tb and not self._is_relevant_tb_level(tb):
-            length += 1
+            prev = tb
             tb = tb.tb_next
-        return length
+        if prev is not None:
+            prev.tb_next = None
 
     def __repr__(self):
         return ("<%s run=%i errors=%i failures=%i>" %

diff --git a/Lib/unittest/test/test_result.py b/Lib/unittest/test/test_result.py
@@ -424,6 +424,47 @@ class tb_frame(object):
         Frame.tb_frame.f_globals['__unittest'] = True
         self.assertTrue(result._is_relevant_tb_level(Frame))
 
+    def testRemoveUnittestTbFrames(self):
+        try:
+            self.fail('too bad')
+        except:
+            exc_info = sys.exc_info()
+        tb = exc_info[2]
+        self.assertEqual(len(list(traceback.walk_tb(tb))), 2)
+        result = unittest.TestResult()
+        result._remove_unittest_tb_frames(tb)
+        self.assertEqual(len(list(traceback.walk_tb(tb))), 1)
+
+    def testExcInfoStringChained(self):  # bpo 49563
+        def raiseAnException():
+            raise ValueError(42)
+        try:
+            try:
+                raiseAnException()
+            except:
+                self.fail('too bad')
+        except:
+            exc_info = sys.exc_info()
+
+        class ResultWithoutFilter(unittest.TestResult):
+            def _remove_unittest_tb_frames(self, tb):
+                pass
+
+        result = ResultWithoutFilter()
        exc_str_no_filter = result._exc_info_to_string(exc_info, self)
+
+        result = unittest.TestResult()
+        exc_str_filtered = result._exc_info_to_string(exc_info, self)
+        for exc_str in [exc_str_no_filter, exc_str_filtered]:
+            self.assertIn('raiseAnException()', exc_str)
+            self.assertIn('raise ValueError(42)', exc_str)
+            self.assertIn("self.fail('too bad')", exc_str)
+            self.assertIn('AssertionError: too bad', exc_str)
+
+        # The unittest call:
+        self.assertNotIn('raise self.failureException(msg)', exc_str_filtered)
+        self.assertIn('raise self.failureException(msg)', exc_str_no_filter)
+
     def testFailFast(self):
         result = unittest.TestResult()
         result._exc_info_to_string = lambda *_: ''