bpo-45292: [PEP 654] Update traceback display code to work with exception groups (GH-29207)

2 files changed, 642 insertions, 14 deletions

diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 1c7db9d..d88851d 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -987,6 +987,35 @@ class TracebackFormatTests(unittest.TestCase):
             self.assertIn('UnhashableException: ex2', tb[4])
             self.assertIn('UnhashableException: ex1', tb[12])
 
+    def deep_eg(self):
+        e = TypeError(1)
+        for i in range(2000):
+            e = ExceptionGroup('eg', [e])
+        return e
+
+    @cpython_only
+    def test_exception_group_deep_recursion_capi(self):
+        from _testcapi import exception_print
+        LIMIT = 75
+        eg = self.deep_eg()
+        with captured_output("stderr") as stderr_f:
+            with support.infinite_recursion(max_depth=LIMIT):
+                exception_print(eg)
+        output = stderr_f.getvalue()
+        self.assertIn('ExceptionGroup', output)
+        self.assertLessEqual(output.count('ExceptionGroup'), LIMIT)
+
+    def test_exception_group_deep_recursion_traceback(self):
+        LIMIT = 75
+        eg = self.deep_eg()
+        with captured_output("stderr") as stderr_f:
+            with support.infinite_recursion(max_depth=LIMIT):
+                traceback.print_exception(type(eg), eg, eg.__traceback__)
+        output = stderr_f.getvalue()
+        self.assertIn('ExceptionGroup', output)
+        self.assertLessEqual(output.count('ExceptionGroup'), LIMIT)
+
+
 cause_message = (
     "\nThe above exception was the direct cause "
     "of the following exception:\n\n")
@@ -998,7 +1027,6 @@ context_message = (
 boundaries = re.compile(
     '(%s|%s)' % (re.escape(cause_message), re.escape(context_message)))
 
-
 class BaseExceptionReportingTests:
 
     def get_exception(self, exception_or_callable):
@@ -1009,6 +1037,8 @@ class BaseExceptionReportingTests:
         except Exception as e:
             return e
 
+    callable_line = get_exception.__code__.co_firstlineno + 4
+
     def zero_div(self):
         1/0 # In zero_div
 
@@ -1234,6 +1264,298 @@ class BaseExceptionReportingTests:
         self.assertEqual(err, f"{str_name}: {str_value}\n")
 
 
+    # #### Exception Groups ####
+
+    def test_exception_group_basic(self):
+        def exc():
+            raise ExceptionGroup("eg", [ValueError(1), TypeError(2)])
+
+        expected = (
+             f'  + Exception Group Traceback (most recent call last):\n'
+             f'  |   File "{__file__}", line {self.callable_line}, in get_exception\n'
+             f'  |     exception_or_callable()\n'
+             f'  |     ^^^^^^^^^^^^^^^^^^^^^^^\n'
+             f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 1}, in exc\n'
+             f'  |     raise ExceptionGroup("eg", [ValueError(1), TypeError(2)])\n'
+             f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+             f'  | ExceptionGroup: eg\n'
+             f'  +-+---------------- 1 ----------------\n'
+             f'    | ValueError: 1\n'
+             f'    +---------------- 2 ----------------\n'
+             f'    | TypeError: 2\n'
+             f'    +------------------------------------\n')
+
+        report = self.get_report(exc)
+        self.assertEqual(report, expected)
+
+    def test_exception_group_cause(self):
+        def exc():
+            EG = ExceptionGroup
+            try:
+                raise EG("eg1", [ValueError(1), TypeError(2)])
+            except Exception as e:
+                raise EG("eg2", [ValueError(3), TypeError(4)]) from e
+
+        expected = (f'  + Exception Group Traceback (most recent call last):\n'
+                    f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 3}, in exc\n'
+                    f'  |     raise EG("eg1", [ValueError(1), TypeError(2)])\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  | ExceptionGroup: eg1\n'
+                    f'  +-+---------------- 1 ----------------\n'
+                    f'    | ValueError: 1\n'
+                    f'    +---------------- 2 ----------------\n'
+                    f'    | TypeError: 2\n'
+                    f'    +------------------------------------\n'
+                    f'\n'
+                    f'The above exception was the direct cause of the following exception:\n'
+                    f'\n'
+                    f'  + Exception Group Traceback (most recent call last):\n'
+                    f'  |   File "{__file__}", line {self.callable_line}, in get_exception\n'
+                    f'  |     exception_or_callable()\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 5}, in exc\n'
+                    f'  |     raise EG("eg2", [ValueError(3), TypeError(4)]) from e\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  | ExceptionGroup: eg2\n'
+                    f'  +-+---------------- 1 ----------------\n'
+                    f'    | ValueError: 3\n'
+                    f'    +---------------- 2 ----------------\n'
+                    f'    | TypeError: 4\n'
+                    f'    +------------------------------------\n')
+
+        report = self.get_report(exc)
+        self.assertEqual(report, expected)
+
+    def test_exception_group_context_with_context(self):
+        def exc():
+            EG = ExceptionGroup
+            try:
+                try:
+                    raise EG("eg1", [ValueError(1), TypeError(2)])
+                except:
+                    raise EG("eg2", [ValueError(3), TypeError(4)])
+            except:
+                raise ImportError(5)
+
+        expected = (
+             f'  + Exception Group Traceback (most recent call last):\n'
+             f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 4}, in exc\n'
+             f'  |     raise EG("eg1", [ValueError(1), TypeError(2)])\n'
+             f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+             f'  | ExceptionGroup: eg1\n'
+             f'  +-+---------------- 1 ----------------\n'
+             f'    | ValueError: 1\n'
+             f'    +---------------- 2 ----------------\n'
+             f'    | TypeError: 2\n'
+             f'    +------------------------------------\n'
+             f'\n'
+             f'During handling of the above exception, another exception occurred:\n'
+             f'\n'
+             f'  + Exception Group Traceback (most recent call last):\n'
+             f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 6}, in exc\n'
+             f'  |     raise EG("eg2", [ValueError(3), TypeError(4)])\n'
+             f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+             f'  | ExceptionGroup: eg2\n'
+             f'  +-+---------------- 1 ----------------\n'
+             f'    | ValueError: 3\n'
+             f'    +---------------- 2 ----------------\n'
+             f'    | TypeError: 4\n'
+             f'    +------------------------------------\n'
+             f'\n'
+             f'During handling of the above exception, another exception occurred:\n'
+             f'\n'
+             f'Traceback (most recent call last):\n'
+             f'  File "{__file__}", line {self.callable_line}, in get_exception\n'
+             f'    exception_or_callable()\n'
+             f'    ^^^^^^^^^^^^^^^^^^^^^^^\n'
+             f'  File "{__file__}", line {exc.__code__.co_firstlineno + 8}, in exc\n'
+             f'    raise ImportError(5)\n'
+             f'    ^^^^^^^^^^^^^^^^^^^^\n'
+             f'ImportError: 5\n')
+
+        report = self.get_report(exc)
+        self.assertEqual(report, expected)
+
+    def test_exception_group_nested(self):
+        def exc():
+            EG = ExceptionGroup
+            VE = ValueError
+            TE = TypeError
+            try:
+                try:
+                    raise EG("nested", [TE(2), TE(3)])
+                except Exception as e:
+                    exc = e
+                raise EG("eg", [VE(1), exc, VE(4)])
+            except:
+                raise EG("top", [VE(5)])
+
+        expected = (f'  + Exception Group Traceback (most recent call last):\n'
+                    f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 9}, in exc\n'
+                    f'  |     raise EG("eg", [VE(1), exc, VE(4)])\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  | ExceptionGroup: eg\n'
+                    f'  +-+---------------- 1 ----------------\n'
+                    f'    | ValueError: 1\n'
+                    f'    +---------------- 2 ----------------\n'
+                    f'    | Exception Group Traceback (most recent call last):\n'
+                    f'    |   File "{__file__}", line {exc.__code__.co_firstlineno + 6}, in exc\n'
+                    f'    |     raise EG("nested", [TE(2), TE(3)])\n'
+                    f'    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'    | ExceptionGroup: nested\n'
+                    f'    +-+---------------- 1 ----------------\n'
+                    f'      | TypeError: 2\n'
+                    f'      +---------------- 2 ----------------\n'
+                    f'      | TypeError: 3\n'
+                    f'      +------------------------------------\n'
+                    f'    +---------------- 3 ----------------\n'
+                    f'    | ValueError: 4\n'
+                    f'    +------------------------------------\n'
+                    f'\n'
+                    f'During handling of the above exception, another exception occurred:\n'
+                    f'\n'
+                    f'  + Exception Group Traceback (most recent call last):\n'
+                    f'  |   File "{__file__}", line {self.callable_line}, in get_exception\n'
+                    f'  |     exception_or_callable()\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  |   File "{__file__}", line {exc.__code__.co_firstlineno + 11}, in exc\n'
+                    f'  |     raise EG("top", [VE(5)])\n'
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^\n'
+                    f'  | ExceptionGroup: top\n'
+                    f'  +-+---------------- 1 ----------------\n'
+                    f'    | ValueError: 5\n'
+                    f'    +------------------------------------\n')
+
+        report = self.get_report(exc)
+        self.assertEqual(report, expected)
+
+    def test_exception_group_width_limit(self):
+        excs = []
+        for i in range(1000):
+            excs.append(ValueError(i))
+        eg = ExceptionGroup('eg', excs)
+
+        expected = ('  | ExceptionGroup: eg\n'
+                    '  +-+---------------- 1 ----------------\n'
+                    '    | ValueError: 0\n'
+                    '    +---------------- 2 ----------------\n'
+                    '    | ValueError: 1\n'
+                    '    +---------------- 3 ----------------\n'
+                    '    | ValueError: 2\n'
+                    '    +---------------- 4 ----------------\n'
+                    '    | ValueError: 3\n'
+                    '    +---------------- 5 ----------------\n'
+                    '    | ValueError: 4\n'
+                    '    +---------------- 6 ----------------\n'
+                    '    | ValueError: 5\n'
+                    '    +---------------- 7 ----------------\n'
+                    '    | ValueError: 6\n'
+                    '    +---------------- 8 ----------------\n'
+                    '    | ValueError: 7\n'
+                    '    +---------------- 9 ----------------\n'
+                    '    | ValueError: 8\n'
+                    '    +---------------- 10 ----------------\n'
+                    '    | ValueError: 9\n'
+                    '    +---------------- 11 ----------------\n'
+                    '    | ValueError: 10\n'
+                    '    +---------------- 12 ----------------\n'
+                    '    | ValueError: 11\n'
+                    '    +---------------- 13 ----------------\n'
+                    '    | ValueError: 12\n'
+                    '    +---------------- 14 ----------------\n'
+                    '    | ValueError: 13\n'
+                    '    +---------------- 15 ----------------\n'
+                    '    | ValueError: 14\n'
+                    '    +---------------- ... ----------------\n'
+                    '    | and 985 more exceptions\n'
+                    '    +------------------------------------\n')
+
+        report = self.get_report(eg)
+        self.assertEqual(report, expected)
+
+    def test_exception_group_depth_limit(self):
+        exc = TypeError('bad type')
+        for i in range(1000):
+            exc = ExceptionGroup(
+                f'eg{i}',
+                [ValueError(i), exc, ValueError(-i)])
+
+        expected = ('  | ExceptionGroup: eg999\n'
+                    '  +-+---------------- 1 ----------------\n'
+                    '    | ValueError: 999\n'
+                    '    +---------------- 2 ----------------\n'
+                    '    | ExceptionGroup: eg998\n'
+                    '    +-+---------------- 1 ----------------\n'
+                    '      | ValueError: 998\n'
+                    '      +---------------- 2 ----------------\n'
+                    '      | ExceptionGroup: eg997\n'
+                    '      +-+---------------- 1 ----------------\n'
+                    '        | ValueError: 997\n'
+                    '        +---------------- 2 ----------------\n'
+                    '        | ExceptionGroup: eg996\n'
+                    '        +-+---------------- 1 ----------------\n'
+                    '          | ValueError: 996\n'
+                    '          +---------------- 2 ----------------\n'
+                    '          | ExceptionGroup: eg995\n'
+                    '          +-+---------------- 1 ----------------\n'
+                    '            | ValueError: 995\n'
+                    '            +---------------- 2 ----------------\n'
+                    '            | ExceptionGroup: eg994\n'
+                    '            +-+---------------- 1 ----------------\n'
+                    '              | ValueError: 994\n'
+                    '              +---------------- 2 ----------------\n'
+                    '              | ExceptionGroup: eg993\n'
+                    '              +-+---------------- 1 ----------------\n'
+                    '                | ValueError: 993\n'
+                    '                +---------------- 2 ----------------\n'
+                    '                | ExceptionGroup: eg992\n'
+                    '                +-+---------------- 1 ----------------\n'
+                    '                  | ValueError: 992\n'
+                    '                  +---------------- 2 ----------------\n'
+                    '                  | ExceptionGroup: eg991\n'
+                    '                  +-+---------------- 1 ----------------\n'
+                    '                    | ValueError: 991\n'
+                    '                    +---------------- 2 ----------------\n'
+                    '                    | ExceptionGroup: eg990\n'
+                    '                    +-+---------------- 1 ----------------\n'
+                    '                      | ValueError: 990\n'
+                    '                      +---------------- 2 ----------------\n'
+                    '                      | ... (max_group_depth is 10)\n'
+                    '                      +---------------- 3 ----------------\n'
+                    '                      | ValueError: -990\n'
+                    '                      +------------------------------------\n'
+                    '                    +---------------- 3 ----------------\n'
+                    '                    | ValueError: -991\n'
+                    '                    +------------------------------------\n'
+                    '                  +---------------- 3 ----------------\n'
+                    '                  | ValueError: -992\n'
+                    '                  +------------------------------------\n'
+                    '                +---------------- 3 ----------------\n'
+                    '                | ValueError: -993\n'
+                    '                +------------------------------------\n'
+                    '              +---------------- 3 ----------------\n'
+                    '              | ValueError: -994\n'
+                    '              +------------------------------------\n'
+                    '            +---------------- 3 ----------------\n'
+                    '            | ValueError: -995\n'
+                    '            +------------------------------------\n'
+                    '          +---------------- 3 ----------------\n'
+                    '          | ValueError: -996\n'
+                    '          +------------------------------------\n'
+                    '        +---------------- 3 ----------------\n'
+                    '        | ValueError: -997\n'
+                    '        +------------------------------------\n'
+                    '      +---------------- 3 ----------------\n'
+                    '      | ValueError: -998\n'
+                    '      +------------------------------------\n'
+                    '    +---------------- 3 ----------------\n'
+                    '    | ValueError: -999\n'
+                    '    +------------------------------------\n')
+
+        report = self.get_report(exc)
+        self.assertEqual(report, expected)
+
+
 class PyExcReportingTests(BaseExceptionReportingTests, unittest.TestCase):
     #
     # This checks reporting through the 'traceback' module, with both
@@ -1913,6 +2235,197 @@ class TestTracebackException(unittest.TestCase):
              ''])
 
 
+class TestTracebackException_ExceptionGroups(unittest.TestCase):
+    def setUp(self):
+        super().setUp()
+        self.eg_info = self._get_exception_group()
+
+    def _get_exception_group(self):
+        def f():
+            1/0
+
+        def g(v):
+            raise ValueError(v)
+
+        self.lno_f = f.__code__.co_firstlineno
+        self.lno_g = g.__code__.co_firstlineno
+
+        try:
+            try:
+                try:
+                    f()
+                except Exception as e:
+                    exc1 = e
+                try:
+                    g(42)
+                except Exception as e:
+                    exc2 = e
+                raise ExceptionGroup("eg1", [exc1, exc2])
+            except ExceptionGroup as e:
+                exc3 = e
+            try:
+                g(24)
+            except Exception as e:
+                exc4 = e
+            raise ExceptionGroup("eg2", [exc3, exc4])
+        except ExceptionGroup:
+            return sys.exc_info()
+        self.fail('Exception Not Raised')
+
+    def test_exception_group_construction(self):
+        eg_info = self.eg_info
+        teg1 = traceback.TracebackException(*eg_info)
+        teg2 = traceback.TracebackException.from_exception(eg_info[1])
+        self.assertIsNot(teg1, teg2)
+        self.assertEqual(teg1, teg2)
+
+    def test_exception_group_format_exception_only(self):
+        teg = traceback.TracebackException(*self.eg_info)
+        formatted = ''.join(teg.format_exception_only()).split('\n')
+        expected = "ExceptionGroup: eg2\n".split('\n')
+
+        self.assertEqual(formatted, expected)
+
+    def test_exception_group_format(self):
+        teg = traceback.TracebackException(*self.eg_info)
+
+        formatted = ''.join(teg.format()).split('\n')
+        lno_f = self.lno_f
+        lno_g = self.lno_g
+
+        expected = [
+                    f'  + Exception Group Traceback (most recent call last):',
+                    f'  |   File "{__file__}", line {lno_g+23}, in _get_exception_group',
+                    f'  |     raise ExceptionGroup("eg2", [exc3, exc4])',
+                    f'  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^',
+                    f'  | ExceptionGroup: eg2',
+                    f'  +-+---------------- 1 ----------------',
+                    f'    | Exception Group Traceback (most recent call last):',
+                    f'    |   File "{__file__}", line {lno_g+16}, in _get_exception_group',
+                    f'    |     raise ExceptionGroup("eg1", [exc1, exc2])',
+                    f'    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^',
+                    f'    | ExceptionGroup: eg1',
+                    f'    +-+---------------- 1 ----------------',
+                    f'      | Traceback (most recent call last):',
+                    f'      |   File "{__file__}", line {lno_g+9}, in _get_exception_group',
+                    f'      |     f()',
+                    f'      |     ^^^',
+                    f'      |   File "{__file__}", line {lno_f+1}, in f',
+                    f'      |     1/0',
+                    f'      |     ~^~',
+                    f'      | ZeroDivisionError: division by zero',
+                    f'      +---------------- 2 ----------------',
+                    f'      | Traceback (most recent call last):',
+                    f'      |   File "{__file__}", line {lno_g+13}, in _get_exception_group',
+                    f'      |     g(42)',
+                    f'      |     ^^^^^',
+                    f'      |   File "{__file__}", line {lno_g+1}, in g',
+                    f'      |     raise ValueError(v)',
+                    f'      |     ^^^^^^^^^^^^^^^^^^^',
+                    f'      | ValueError: 42',
+                    f'      +------------------------------------',
+                    f'    +---------------- 2 ----------------',
+                    f'    | Traceback (most recent call last):',
+                    f'    |   File "{__file__}", line {lno_g+20}, in _get_exception_group',
+                    f'    |     g(24)',
+                    f'    |     ^^^^^',
+                    f'    |   File "{__file__}", line {lno_g+1}, in g',
+                    f'    |     raise ValueError(v)',
+                    f'    |     ^^^^^^^^^^^^^^^^^^^',
+                    f'    | ValueError: 24',
+                    f'    +------------------------------------',
+                    f'']
+
+        self.assertEqual(formatted, expected)
+
+    def test_max_group_width(self):
+        excs1 = []
+        excs2 = []
+        for i in range(3):
+            excs1.append(ValueError(i))
+        for i in range(10):
+            excs2.append(TypeError(i))
+
+        EG = ExceptionGroup
+        eg = EG('eg', [EG('eg1', excs1), EG('eg2', excs2)])
+
+        teg = traceback.TracebackException.from_exception(eg, max_group_width=2)
+        formatted = ''.join(teg.format()).split('\n')
+
+        expected = [
+                    f'  | ExceptionGroup: eg',
+                    f'  +-+---------------- 1 ----------------',
+                    f'    | ExceptionGroup: eg1',
+                    f'    +-+---------------- 1 ----------------',
+                    f'      | ValueError: 0',
+                    f'      +---------------- 2 ----------------',
+                    f'      | ValueError: 1',
+                    f'      +---------------- ... ----------------',
+                    f'      | and 1 more exception',
+                    f'      +------------------------------------',
+                    f'    +---------------- 2 ----------------',
+                    f'    | ExceptionGroup: eg2',
+                    f'    +-+---------------- 1 ----------------',
+                    f'      | TypeError: 0',
+                    f'      +---------------- 2 ----------------',
+                    f'      | TypeError: 1',
+                    f'      +---------------- ... ----------------',
+                    f'      | and 8 more exceptions',
+                    f'      +------------------------------------',
+                    f'']
+
+        self.assertEqual(formatted, expected)
+
+    def test_max_group_depth(self):
+        exc = TypeError('bad type')
+        for i in range(3):
+            exc = ExceptionGroup('exc', [ValueError(-i), exc, ValueError(i)])
+
+        teg = traceback.TracebackException.from_exception(exc, max_group_depth=2)
+        formatted = ''.join(teg.format()).split('\n')
+
+        expected = [
+                    f'  | ExceptionGroup: exc',
+                    f'  +-+---------------- 1 ----------------',
+                    f'    | ValueError: -2',
+                    f'    +---------------- 2 ----------------',
+                    f'    | ExceptionGroup: exc',
+                    f'    +-+---------------- 1 ----------------',
+                    f'      | ValueError: -1',
+                    f'      +---------------- 2 ----------------',
+                    f'      | ... (max_group_depth is 2)',
+                    f'      +---------------- 3 ----------------',
+                    f'      | ValueError: 1',
+                    f'      +------------------------------------',
+                    f'    +---------------- 3 ----------------',
+                    f'    | ValueError: 2',
+                    f'    +------------------------------------',
+                    f'']
+
+        self.assertEqual(formatted, expected)
+
+    def test_comparison(self):
+        try:
+            raise self.eg_info[1]
+        except ExceptionGroup:
+            exc_info = sys.exc_info()
+        for _ in range(5):
+            try:
+                raise exc_info[1]
+            except:
+                exc_info = sys.exc_info()
+        exc = traceback.TracebackException(*exc_info)
+        exc2 = traceback.TracebackException(*exc_info)
+        exc3 = traceback.TracebackException(*exc_info, limit=300)
+        ne = traceback.TracebackException(*exc_info, limit=3)
+        self.assertIsNot(exc, exc2)
+        self.assertEqual(exc, exc2)
+        self.assertEqual(exc, exc3)
+        self.assertNotEqual(exc, ne)
+        self.assertNotEqual(exc, object())
+        self.assertEqual(exc, ALWAYS_EQ)
+
+
 class MiscTest(unittest.TestCase):
 
     def test_all(self):
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 568f3ff..97caa13 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -4,6 +4,7 @@ import collections
 import itertools
 import linecache
 import sys
+import textwrap
 from contextlib import suppress
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
@@ -601,6 +602,29 @@ def _extract_caret_anchors_from_line_segment(segment):
     return None
 
 
+class _ExceptionPrintContext:
+    def __init__(self):
+        self.seen = set()
+        self.exception_group_depth = 0
+        self.need_close = False
+
+    def indent(self):
+        return ' ' * (2 * self.exception_group_depth)
+
+    def emit(self, text_gen, margin_char=None):
+        if margin_char is None:
+            margin_char = '|'
+        indent_str = self.indent()
+        if self.exception_group_depth:
+            indent_str += margin_char + ' '
+
+        if isinstance(text_gen, str):
+            yield textwrap.indent(text_gen, indent_str, lambda line: True)
+        else:
+            for text in text_gen:
+                yield textwrap.indent(text, indent_str, lambda line: True)
+
+
 class TracebackException:
     """An exception ready for rendering.
 
@@ -608,6 +632,11 @@ class TracebackException:
     to this intermediary form to ensure that no references are held, while
     still being able to fully print or format it.
 
+    max_group_width and max_group_depth control the formatting of exception
+    groups. The depth refers to the nesting level of the group, and the width
+    refers to the size of a single exception group's exceptions array. The
+    formatted output is truncated when either limit is exceeded.
+
     Use `from_exception` to create TracebackException instances from exception
     objects, or the constructor to create TracebackException instances from
     individual components.
@@ -635,7 +664,7 @@ class TracebackException:
 
     def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
             lookup_lines=True, capture_locals=False, compact=False,
-            _seen=None):
+            max_group_width=15, max_group_depth=10, _seen=None):
         # NB: we need to accept exc_traceback, exc_value, exc_traceback to
         # permit backwards compat with the existing API, otherwise we
         # need stub thunk objects just to glue it together.
@@ -645,7 +674,9 @@ class TracebackException:
             _seen = set()
         _seen.add(id(exc_value))
 
-        # TODO: locals.
+        self.max_group_width = max_group_width
+        self.max_group_depth = max_group_depth
+
         self.stack = StackSummary._extract_from_extended_frame_gen(
             _walk_tb_with_full_positions(exc_traceback),
             limit=limit, lookup_lines=lookup_lines,
@@ -685,6 +716,8 @@ class TracebackException:
                         limit=limit,
                         lookup_lines=lookup_lines,
                         capture_locals=capture_locals,
+                        max_group_width=max_group_width,
+                        max_group_depth=max_group_depth,
                         _seen=_seen)
                 else:
                     cause = None
@@ -704,15 +737,38 @@ class TracebackException:
                         limit=limit,
                         lookup_lines=lookup_lines,
                         capture_locals=capture_locals,
+                        max_group_width=max_group_width,
+                        max_group_depth=max_group_depth,
                         _seen=_seen)
                 else:
                     context = None
+
+                if e and isinstance(e, BaseExceptionGroup):
+                    exceptions = []
+                    for exc in e.exceptions:
+                        texc = TracebackException(
+                            type(exc),
+                            exc,
+                            exc.__traceback__,
+                            limit=limit,
+                            lookup_lines=lookup_lines,
+                            capture_locals=capture_locals,
+                            max_group_width=max_group_width,
+                            max_group_depth=max_group_depth,
+                            _seen=_seen)
+                        exceptions.append(texc)
+                else:
+                    exceptions = None
+
                 te.__cause__ = cause
                 te.__context__ = context
+                te.exceptions = exceptions
                 if cause:
                     queue.append((te.__cause__, e.__cause__))
                 if context:
                     queue.append((te.__context__, e.__context__))
+                if exceptions:
+                    queue.extend(zip(te.exceptions, e.exceptions))
 
     @classmethod
     def from_exception(cls, exc, *args, **kwargs):
@@ -795,7 +851,7 @@ class TracebackException:
         msg = self.msg or "<no detail available>"
         yield "{}: {}{}\n".format(stype, msg, filename_suffix)
 
-    def format(self, *, chain=True):
+    def format(self, *, chain=True, _ctx=None):
         """Format the exception.
 
         If chain is not *True*, *__cause__* and *__context__* will not be formatted.
@@ -808,10 +864,13 @@ class TracebackException:
         string in the output.
         """
 
+        if _ctx is None:
+            _ctx = _ExceptionPrintContext()
+
         output = []
         exc = self
-        while exc:
-            if chain:
+        if chain:
+            while exc:
                 if exc.__cause__ is not None:
                     chained_msg = _cause_message
                     chained_exc = exc.__cause__
@@ -825,17 +884,73 @@ class TracebackException:
 
                 output.append((chained_msg, exc))
                 exc = chained_exc
-            else:
-                output.append((None, exc))
-                exc = None
+        else:
+            output.append((None, exc))
 
         for msg, exc in reversed(output):
             if msg is not None:
-                yield msg
-            if exc.stack:
-                yield 'Traceback (most recent call last):\n'
-                yield from exc.stack.format()
-            yield from exc.format_exception_only()
+                yield from _ctx.emit(msg)
+            if exc.exceptions is None:
+                if exc.stack:
+                    yield from _ctx.emit('Traceback (most recent call last):\n')
+                    yield from _ctx.emit(exc.stack.format())
+                yield from _ctx.emit(exc.format_exception_only())
+            elif _ctx.exception_group_depth > self.max_group_depth:
+                # exception group, but depth exceeds limit
+                yield from _ctx.emit(
+                    f"... (max_group_depth is {self.max_group_depth})\n")
+            else:
+                # format exception group
+                is_toplevel = (_ctx.exception_group_depth == 0)
+                if is_toplevel:
+                     _ctx.exception_group_depth += 1
+
+                if exc.stack:
+                    yield from _ctx.emit(
+                        'Exception Group Traceback (most recent call last):\n',
+                        margin_char = '+' if is_toplevel else None)
+                    yield from _ctx.emit(exc.stack.format())
+
+                yield from _ctx.emit(exc.format_exception_only())
+                num_excs = len(exc.exceptions)
+                if num_excs <= self.max_group_width:
+                    n = num_excs
+                else:
+                    n = self.max_group_width + 1
+                _ctx.need_close = False
+                for i in range(n):
+                    last_exc = (i == n-1)
+                    if last_exc:
+                        # The closing frame may be added by a recursive call
+                        _ctx.need_close = True
+
+                    if self.max_group_width is not None:
+                        truncated = (i >= self.max_group_width)
+                    else:
+                        truncated = False
+                    title = f'{i+1}' if not truncated else '...'
+                    yield (_ctx.indent() +
+                           ('+-' if i==0 else '  ') +
+                           f'+---------------- {title} ----------------\n')
+                    _ctx.exception_group_depth += 1
+                    if not truncated:
+                        yield from exc.exceptions[i].format(chain=chain, _ctx=_ctx)
+                    else:
+                        remaining = num_excs - self.max_group_width
+                        plural = 's' if remaining > 1 else ''
+                        yield from _ctx.emit(
+                            f"and {remaining} more exception{plural}\n")
+
+                    if last_exc and _ctx.need_close:
+                        yield (_ctx.indent() +
+                               "+------------------------------------\n")
+                        _ctx.need_close = False
+                    _ctx.exception_group_depth -= 1
+
+                if is_toplevel:
+                    assert _ctx.exception_group_depth == 1
+                    _ctx.exception_group_depth = 0
+
 
     def print(self, *, file=None, chain=True):
         """Print the result of self.format(chain=chain) to 'file'."""