fix-traceback-in-plain-mode

Author: Darshan808

IPython/core/ultratb.py

@@ -364,11 +364,7 @@ def _format_exception_only(
                             )
                         )
 
-            try:
-                assert hasattr(value, "msg")
-                s = value.msg
-            except Exception:
-                s = self._some_str(value)
+            s = self._some_str(value)
             if s:
                 output_list.append(
                     theme_table[self._theme_name].format(

tests/test_ultratb.py

@@ -437,6 +437,20 @@ def test_plain_reports_notes(self):
             ip.run_cell("%xmode Verbose")
 
 
+class ExceptionMessagePreferenceTest(unittest.TestCase):
+    """
+    Test that exception string representation is preferred over .msg attribute
+    for non-SyntaxError exceptions in %xmode plain.
+    """
+
+    def test_jsondecodeerror_message(self):
+        cell = "import json;json.loads('{\"a\": }')"
+        expected = "JSONDecodeError: Expecting value: line 1 column 7 (char 6)"
+        ip.run_cell("%xmode plain")
+        with tt.AssertPrints(expected):
+            ip.run_cell(cell)
+        ip.run_cell("%xmode context")
+
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