gh-105292: Add option to make traceback.TracebackException.format_exception_only recurse into exception groups (#105294)

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

2 files changed, 26 insertions, 6 deletions

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 19a2be8..da7d1fb 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -2792,6 +2792,20 @@ class TestTracebackException_ExceptionGroups(unittest.TestCase):
 
         self.assertEqual(formatted, expected)
 
+    def test_exception_group_format_exception_onlyi_recursive(self):
+        teg = traceback.TracebackException.from_exception(self.eg)
+        formatted = ''.join(teg.format_exception_only(show_group=True)).split('\n')
+        expected = [
+                     'ExceptionGroup: eg2 (2 sub-exceptions)',
+                     '   ExceptionGroup: eg1 (2 sub-exceptions)',
+                     '      ZeroDivisionError: division by zero',
+                     '      ValueError: 42',
+                     '   ValueError: 24',
+                     ''
+                   ]
+
+        self.assertEqual(formatted, expected)
+
     def test_exception_group_format(self):
         teg = traceback.TracebackException.from_exception(self.eg)
 
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 21e3204..354754b 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -826,7 +826,7 @@ class TracebackException:
     def __str__(self):
         return self._str
 
-    def format_exception_only(self):
+    def format_exception_only(self, *, show_group=False, _depth=0):
         """Format the exception part of the traceback.
 
         The return value is a generator of strings, each ending in a newline.
@@ -839,8 +839,10 @@ class TracebackException:
         The message indicating which exception occurred is always the last
         string in the output.
         """
+
+        indent = 3 * _depth * ' '
         if self.exc_type is None:
-            yield _format_final_exc_line(None, self._str)
+            yield indent + _format_final_exc_line(None, self._str)
             return
 
         stype = self.exc_type.__qualname__
@@ -851,9 +853,9 @@ class TracebackException:
             stype = smod + '.' + stype
 
         if not issubclass(self.exc_type, SyntaxError):
-            yield _format_final_exc_line(stype, self._str)
+            yield indent + _format_final_exc_line(stype, self._str)
         else:
-            yield from self._format_syntax_error(stype)
+            yield from [indent + l for l in self._format_syntax_error(stype)]
 
         if (
             isinstance(self.__notes__, collections.abc.Sequence)
@@ -861,9 +863,13 @@ class TracebackException:
         ):
             for note in self.__notes__:
                 note = _safe_string(note, 'note')
-                yield from [l + '\n' for l in note.split('\n')]
+                yield from [indent + l + '\n' for l in note.split('\n')]
         elif self.__notes__ is not None:
-            yield "{}\n".format(_safe_string(self.__notes__, '__notes__', func=repr))
+            yield indent + "{}\n".format(_safe_string(self.__notes__, '__notes__', func=repr))
+
+        if self.exceptions and show_group:
+            for ex in self.exceptions:
+                yield from ex.format_exception_only(show_group=show_group, _depth=_depth+1)
 
     def _format_syntax_error(self, stype):
         """Format SyntaxError exceptions (internal helper)."""