Dec 7, 2025 · Jan 25, 2026 · Jan 30, 2026 · Feb 3, 2026 · Feb 3, 2026 · Feb 3, 2026
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
   raises a :exc:`TypeError` if any contained exception is not an
   :exc:`Exception` subclass.

   .. impl-detail::

      The ``excs`` parameter may be any sequence, but lists and tuples are
      specifically processed more efficiently here. For optimal performance,
      pass a tuple as ``excs``.

   .. attribute:: message

       The ``msg`` argument to the constructor. This is a read-only attribute.
diff --git a/Include/cpython/pyerrors.h b/Include/cpython/pyerrors.h
    PyException_HEAD
    PyObject *msg;
    PyObject *excs;
    PyObject *excs_str;
 } PyBaseExceptionGroupObject;

 typedef struct {
diff --git a/Lib/test/test_exception_group.py b/Lib/test/test_exception_group.py
 import collections.abc
 import collections
 import types
 import unittest
 from test.support import skip_emscripten_stack_overflow, skip_wasi_stack_overflow, exceeds_recursion_limit
                      "MyEG('flat', [ValueError(1), TypeError(2)]), "
                      "TypeError(2)])"))

    def test_exceptions_mutation(self):
        class MyEG(ExceptionGroup):
            pass

        excs = [ValueError(1), TypeError(2)]
        eg = MyEG('test', excs)

        self.assertEqual(repr(eg), "MyEG('test', [ValueError(1), TypeError(2)])")
        excs.clear()

        # Ensure that clearing the exceptions sequence doesn't change the repr.
        self.assertEqual(repr(eg), "MyEG('test', [ValueError(1), TypeError(2)])")

        # Ensure that the args are still as passed.
        self.assertEqual(eg.args, ('test', []))

        excs = (ValueError(1), KeyboardInterrupt(2))
        eg = BaseExceptionGroup('test', excs)

        # Ensure that immutable sequences still work fine.
        self.assertEqual(
            repr(eg),
            "BaseExceptionGroup('test', (ValueError(1), KeyboardInterrupt(2)))"
        )

        # Test non-standard custom sequences.
        excs = collections.deque([ValueError(1), TypeError(2)])
        eg = ExceptionGroup('test', excs)

        self.assertEqual(
            repr(eg),
            "ExceptionGroup('test', deque([ValueError(1), TypeError(2)]))"
        )
        excs.clear()

        # Ensure that clearing the exceptions sequence doesn't change the repr.
        self.assertEqual(
            repr(eg),
            "ExceptionGroup('test', deque([ValueError(1), TypeError(2)]))"
        )

    def test_repr_raises(self):
        class MySeq(collections.abc.Sequence):
            def __init__(self, raises):
                self.raises = raises

            def __len__(self):
                return 1

            def __getitem__(self, index):
                if index == 0:
                    return ValueError(1)
                raise IndexError

            def __repr__(self):
                if self.raises:
                    raise self.raises
                return None

        seq = MySeq(None)
        with self.assertRaisesRegex(
            TypeError,
            r"__repr__ returned non-string \(type NoneType\)"
        ):
            ExceptionGroup("test", seq)

        seq = MySeq(ValueError)
        with self.assertRaises(ValueError):
            BaseExceptionGroup("test", seq)



 def create_simple_eg():
    excs = []
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-19-16-40-24.gh-issue-141732.PTetqp.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-19-16-40-24.gh-issue-141732.PTetqp.rst
 Ensure the :meth:`~object.__repr__` for :exc:`ExceptionGroup` and :exc:`BaseExceptionGroup` does
 not change when the exception sequence that was original passed in to its constructor is subsequently mutated.
diff --git a/Objects/exceptions.c b/Objects/exceptions.c

 #define ComplexExtendsException(EXCBASE, EXCNAME, EXCSTORE, EXCNEW, \
                                EXCMETHODS, EXCMEMBERS, EXCGETSET, \
                                EXCSTR, EXCDOC) \
                                EXCSTR,EXCREPR,EXCDOC) \
 static PyTypeObject _PyExc_ ## EXCNAME = { \
    PyVarObject_HEAD_INIT(NULL, 0) \
    # EXCNAME, \
    sizeof(Py ## EXCSTORE ## Object), 0, \
    EXCSTORE ## _dealloc, 0, 0, 0, 0,0, 0, 0, 0, 0, 0, \
    EXCSTORE ## _dealloc, 0, 0, 0, 0,EXCREPR, 0, 0, 0, 0, 0, \
    EXCSTR, 0, 0, 0, \
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC, \
    PyDoc_STR(EXCDOC), EXCSTORE ## _traverse, \
 }

 ComplexExtendsException(PyExc_Exception, StopIteration, StopIteration,
                        0, 0, StopIteration_members, 0, 0,
                        0, 0, StopIteration_members, 0, 0, 0,
                        "Signal the end from iterator.__next__().");


 };

 ComplexExtendsException(PyExc_BaseException, SystemExit, SystemExit,
                        0, 0, SystemExit_members, 0, 0,
                        0, 0, SystemExit_members, 0, 0, 0,
                        "Request to exit from the interpreter.");

 /*

    PyObject *message = NULL;
    PyObject *exceptions = NULL;
    PyObject *exceptions_str = NULL;

    if (!PyArg_ParseTuple(args,
                          "UO:BaseExceptionGroup.__new__",
        return NULL;
    }

    /* Save initial exceptions sequence as a string in case sequence is mutated */
    if (!PyList_Check(exceptions) && !PyTuple_Check(exceptions)) {
        exceptions_str = PyObject_Repr(exceptions);
        if (exceptions_str == NULL) {
            /* We don't hold a reference to exceptions, so clear it before
             * attempting a decref in the cleanup.
             */
            exceptions = NULL;
            goto error;
        }
    }

    exceptions = PySequence_Tuple(exceptions);
    if (!exceptions) {
        return NULL;

    self->msg = Py_NewRef(message);
    self->excs = exceptions;
    self->excs_str = exceptions_str;
    return (PyObject*)self;
 error:
    Py_DECREF(exceptions);
    Py_XDECREF(exceptions);
    Py_XDECREF(exceptions_str);
    return NULL;
 }

    PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
    Py_CLEAR(self->msg);
    Py_CLEAR(self->excs);
    Py_CLEAR(self->excs_str);
    return BaseException_clear(op);
 }

    PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
    Py_VISIT(self->msg);
    Py_VISIT(self->excs);
    Py_VISIT(self->excs_str);
    return BaseException_traverse(op, visit, arg);
 }

        self->msg, num_excs, num_excs > 1 ? "s" : "");
 }

 static PyObject *
 BaseExceptionGroup_repr(PyObject *op)
 {
    PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
    assert(self->msg);

    PyObject *exceptions_str = NULL;

    /* Use the saved exceptions string for custom sequences. */
    if (self->excs_str) {
        exceptions_str = Py_NewRef(self->excs_str);
    }
    else {
        assert(self->excs);

        /* Older versions delegated to BaseException, inserting the current
         * value of self.args[1]; but this can be mutable and go out-of-sync
         * with self.exceptions. Instead, use self.exceptions for accuracy,
         * making it look like self.args[1] for backwards compatibility. */
        if (PyList_Check(PyTuple_GET_ITEM(self->args, 1))) {
            PyObject *exceptions_list = PySequence_List(self->excs);
            if (!exceptions_list) {
                return NULL;
            }

            exceptions_str = PyObject_Repr(exceptions_list);
            Py_DECREF(exceptions_list);
        }
        else {
            exceptions_str = PyObject_Repr(self->excs);
        }

        if (!exceptions_str) {
            return NULL;
        }
    }

    assert(exceptions_str != NULL);

    const char *name = _PyType_Name(Py_TYPE(self));
    PyObject *repr = PyUnicode_FromFormat(
        "%s(%R, %U)", name,
        self->msg, exceptions_str);

    Py_DECREF(exceptions_str);
    return repr;
 }

 /*[clinic input]
 @critical_section
 BaseExceptionGroup.derive
 ComplexExtendsException(PyExc_BaseException, BaseExceptionGroup,
    BaseExceptionGroup, BaseExceptionGroup_new /* new */,
    BaseExceptionGroup_methods, BaseExceptionGroup_members,
    0 /* getset */, BaseExceptionGroup_str,
    0 /* getset */, BaseExceptionGroup_str, BaseExceptionGroup_repr,
    "A combination of multiple unrelated exceptions.");

 /*
 ComplexExtendsException(PyExc_Exception, ImportError,
                        ImportError, 0 /* new */,
                        ImportError_methods, ImportError_members,
                        0 /* getset */, ImportError_str,
                        0 /* getset */, ImportError_str, 0,
                        "Import can't find module, or can't find name in "
                        "module.");

 ComplexExtendsException(PyExc_Exception, OSError,
                        OSError, OSError_new,
                        OSError_methods, OSError_members, OSError_getset,
                        OSError_str,
                        OSError_str, 0,
                        "Base class for I/O related errors.");


 ComplexExtendsException(PyExc_Exception, NameError,
                        NameError, 0,
                        NameError_methods, NameError_members,
                        0, BaseException_str, "Name not found globally.");
                        0, BaseException_str,0,"Name not found globally.");

 /*
 *    UnboundLocalError extends NameError
 ComplexExtendsException(PyExc_Exception, AttributeError,
                        AttributeError, 0,
                        AttributeError_methods, AttributeError_members,
                        0, BaseException_str, "Attribute not found.");
                        0, BaseException_str,0,"Attribute not found.");

 /*
 *    SyntaxError extends Exception

 ComplexExtendsException(PyExc_Exception, SyntaxError, SyntaxError,
                        0, 0, SyntaxError_members, 0,
                        SyntaxError_str, "Invalid syntax.");
                        SyntaxError_str,0,"Invalid syntax.");


 /*
 }

 ComplexExtendsException(PyExc_LookupError, KeyError, BaseException,
                        0, 0, 0, 0, KeyError_str, "Mapping key not found.");
                        0, 0, 0, 0, KeyError_str,0,"Mapping key not found.");


 /*
Original file line number	Diff line number	Diff line change
Expand Up		@@ -971,6 +971,12 @@ their subgroups based on the types of the contained exceptions.
		raises a :exc:`TypeError` if any contained exception is not an
		:exc:`Exception` subclass.

		.. impl-detail::

		The ``excs`` parameter may be any sequence, but lists and tuples are
		specifically processed more efficiently here. For optimal performance,
		pass a tuple as ``excs``.

		.. attribute:: message

		The ``msg`` argument to the constructor. This is a read-only attribute.
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -18,6 +18,7 @@ typedef struct {
		PyException_HEAD
		PyObject *msg;
		PyObject *excs;
		PyObject *excs_str;
		} PyBaseExceptionGroupObject;

		typedef struct {
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -1,4 +1,4 @@
		import collections.abc
		import collections
		import types
		import unittest
		from test.support import skip_emscripten_stack_overflow, skip_wasi_stack_overflow, exceeds_recursion_limit
Expand DownExpand Up		@@ -193,6 +193,77 @@ class MyEG(ExceptionGroup):
		"MyEG('flat', [ValueError(1), TypeError(2)]), "
		"TypeError(2)])"))

		def test_exceptions_mutation(self):
		class MyEG(ExceptionGroup):
		pass

		excs = [ValueError(1), TypeError(2)]
		eg = MyEG('test', excs)

		self.assertEqual(repr(eg), "MyEG('test', [ValueError(1), TypeError(2)])")
		excs.clear()

		# Ensure that clearing the exceptions sequence doesn't change the repr.
		self.assertEqual(repr(eg), "MyEG('test', [ValueError(1), TypeError(2)])")

		# Ensure that the args are still as passed.
		self.assertEqual(eg.args, ('test', []))

		excs = (ValueError(1), KeyboardInterrupt(2))
		eg = BaseExceptionGroup('test', excs)

		# Ensure that immutable sequences still work fine.
		self.assertEqual(
		repr(eg),
		"BaseExceptionGroup('test', (ValueError(1), KeyboardInterrupt(2)))"
		)

		# Test non-standard custom sequences.
		excs = collections.deque([ValueError(1), TypeError(2)])
		eg = ExceptionGroup('test', excs)

		self.assertEqual(
		repr(eg),
		"ExceptionGroup('test', deque([ValueError(1), TypeError(2)]))"
		)
		excs.clear()

		# Ensure that clearing the exceptions sequence doesn't change the repr.
		self.assertEqual(
		repr(eg),
		"ExceptionGroup('test', deque([ValueError(1), TypeError(2)]))"
		)

		def test_repr_raises(self):
		class MySeq(collections.abc.Sequence):
		def __init__(self, raises):
		self.raises = raises

		def __len__(self):
		return 1

		def __getitem__(self, index):
		if index == 0:
		return ValueError(1)
		raise IndexError

		def __repr__(self):
		if self.raises:
		raise self.raises
		return None

		seq = MySeq(None)
		with self.assertRaisesRegex(
		TypeError,
		r"__repr__ returned non-string \(type NoneType\)"
		):
		ExceptionGroup("test", seq)

		seq = MySeq(ValueError)
		with self.assertRaises(ValueError):
		BaseExceptionGroup("test", seq)



		def create_simple_eg():
		excs = []
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Ensure the :meth:`~object.__repr__` for :exc:`ExceptionGroup` and :exc:`BaseExceptionGroup` does
		not change when the exception sequence that was original passed in to its constructor is subsequently mutated.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -695,12 +695,12 @@ PyTypeObject _PyExc_ ## EXCNAME = { \

		#define ComplexExtendsException(EXCBASE, EXCNAME, EXCSTORE, EXCNEW, \
		EXCMETHODS, EXCMEMBERS, EXCGETSET, \
		EXCSTR, EXCDOC) \
		EXCSTR,EXCREPR,EXCDOC) \
		static PyTypeObject _PyExc_ ## EXCNAME = { \
		PyVarObject_HEAD_INIT(NULL, 0) \
		# EXCNAME, \
		sizeof(Py ## EXCSTORE ## Object), 0, \
		EXCSTORE ## _dealloc, 0, 0, 0, 0,0, 0, 0, 0, 0, 0, \
		EXCSTORE ## _dealloc, 0, 0, 0, 0,EXCREPR, 0, 0, 0, 0, 0, \
		EXCSTR, 0, 0, 0, \
		Py_TPFLAGS_DEFAULT \| Py_TPFLAGS_BASETYPE \| Py_TPFLAGS_HAVE_GC, \
		PyDoc_STR(EXCDOC), EXCSTORE ## _traverse, \
Expand DownExpand Up		@@ -793,7 +793,7 @@ StopIteration_traverse(PyObject op, visitproc visit, void arg)
		}

		ComplexExtendsException(PyExc_Exception, StopIteration, StopIteration,
		0, 0, StopIteration_members, 0, 0,
		0, 0, StopIteration_members, 0, 0, 0,
		"Signal the end from iterator.__next__().");


Expand DownExpand Up		@@ -866,7 +866,7 @@ static PyMemberDef SystemExit_members[] = {
		};

		ComplexExtendsException(PyExc_BaseException, SystemExit, SystemExit,
		0, 0, SystemExit_members, 0, 0,
		0, 0, SystemExit_members, 0, 0, 0,
		"Request to exit from the interpreter.");

		/*
Expand All		@@ -891,6 +891,7 @@ BaseExceptionGroup_new(PyTypeObject type, PyObject args, PyObject *kwds)

		PyObject *message = NULL;
		PyObject *exceptions = NULL;
		PyObject *exceptions_str = NULL;

		if (!PyArg_ParseTuple(args,
		"UO:BaseExceptionGroup.__new__",
Expand All		@@ -906,6 +907,18 @@ BaseExceptionGroup_new(PyTypeObject type, PyObject args, PyObject *kwds)
		return NULL;
		}

		/* Save initial exceptions sequence as a string in case sequence is mutated */
		if (!PyList_Check(exceptions) && !PyTuple_Check(exceptions)) {
		exceptions_str = PyObject_Repr(exceptions);
		if (exceptions_str == NULL) {
		/* We don't hold a reference to exceptions, so clear it before
		* attempting a decref in the cleanup.
		*/
		exceptions = NULL;
		goto error;
		}
		}

		exceptions = PySequence_Tuple(exceptions);
		if (!exceptions) {
		return NULL;
Expand DownExpand Up		@@ -989,9 +1002,11 @@ BaseExceptionGroup_new(PyTypeObject type, PyObject args, PyObject *kwds)

		self->msg = Py_NewRef(message);
		self->excs = exceptions;
		self->excs_str = exceptions_str;
		return (PyObject*)self;
		error:
		Py_DECREF(exceptions);
		Py_XDECREF(exceptions);
		Py_XDECREF(exceptions_str);
		return NULL;
		}

Expand DownExpand Up		@@ -1030,6 +1045,7 @@ BaseExceptionGroup_clear(PyObject *op)
		PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
		Py_CLEAR(self->msg);
		Py_CLEAR(self->excs);
		Py_CLEAR(self->excs_str);
		return BaseException_clear(op);
		}

Expand All		@@ -1047,6 +1063,7 @@ BaseExceptionGroup_traverse(PyObject op, visitproc visit, void arg)
		PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
		Py_VISIT(self->msg);
		Py_VISIT(self->excs);
		Py_VISIT(self->excs_str);
		return BaseException_traverse(op, visit, arg);
		}

Expand All		@@ -1064,6 +1081,54 @@ BaseExceptionGroup_str(PyObject *op)
		self->msg, num_excs, num_excs > 1 ? "s" : "");
		}

		static PyObject *
		BaseExceptionGroup_repr(PyObject *op)
		{
		PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
		assert(self->msg);

		PyObject *exceptions_str = NULL;

		/* Use the saved exceptions string for custom sequences. */
		if (self->excs_str) {
		exceptions_str = Py_NewRef(self->excs_str);
		}
		else {
		assert(self->excs);

		/* Older versions delegated to BaseException, inserting the current
		* value of self.args[1]; but this can be mutable and go out-of-sync
		* with self.exceptions. Instead, use self.exceptions for accuracy,
		* making it look like self.args[1] for backwards compatibility. */
		if (PyList_Check(PyTuple_GET_ITEM(self->args, 1))) {
		PyObject *exceptions_list = PySequence_List(self->excs);
		if (!exceptions_list) {
		return NULL;
		}

		exceptions_str = PyObject_Repr(exceptions_list);
		Py_DECREF(exceptions_list);
		}
		else {
		exceptions_str = PyObject_Repr(self->excs);
		}

		if (!exceptions_str) {
		return NULL;
		}
		}

		assert(exceptions_str != NULL);

		const char *name = _PyType_Name(Py_TYPE(self));
		PyObject *repr = PyUnicode_FromFormat(
		"%s(%R, %U)", name,
		self->msg, exceptions_str);

		Py_DECREF(exceptions_str);
		return repr;
		}

		/*[clinic input]
		@critical_section
		BaseExceptionGroup.derive
Expand DownExpand Up		@@ -1698,7 +1763,7 @@ static PyMethodDef BaseExceptionGroup_methods[] = {
		ComplexExtendsException(PyExc_BaseException, BaseExceptionGroup,
		BaseExceptionGroup, BaseExceptionGroup_new /* new */,
		BaseExceptionGroup_methods, BaseExceptionGroup_members,
		0 /* getset */, BaseExceptionGroup_str,
		0 /* getset */, BaseExceptionGroup_str, BaseExceptionGroup_repr,
		"A combination of multiple unrelated exceptions.");

		/*
Expand DownExpand Up		@@ -1884,7 +1949,7 @@ static PyMethodDef ImportError_methods[] = {
		ComplexExtendsException(PyExc_Exception, ImportError,
		ImportError, 0 /* new */,
		ImportError_methods, ImportError_members,
		0 /* getset */, ImportError_str,
		0 /* getset */, ImportError_str, 0,
		"Import can't find module, or can't find name in "
		"module.");

Expand DownExpand Up		@@ -2356,7 +2421,7 @@ static PyGetSetDef OSError_getset[] = {
		ComplexExtendsException(PyExc_Exception, OSError,
		OSError, OSError_new,
		OSError_methods, OSError_members, OSError_getset,
		OSError_str,
		OSError_str, 0,
		"Base class for I/O related errors.");


Expand DownExpand Up		@@ -2497,7 +2562,7 @@ static PyMethodDef NameError_methods[] = {
		ComplexExtendsException(PyExc_Exception, NameError,
		NameError, 0,
		NameError_methods, NameError_members,
		0, BaseException_str, "Name not found globally.");
		0, BaseException_str,0,"Name not found globally.");

		/*
		* UnboundLocalError extends NameError
Expand DownExpand Up		@@ -2631,7 +2696,7 @@ static PyMethodDef AttributeError_methods[] = {
		ComplexExtendsException(PyExc_Exception, AttributeError,
		AttributeError, 0,
		AttributeError_methods, AttributeError_members,
		0, BaseException_str, "Attribute not found.");
		0, BaseException_str,0,"Attribute not found.");

		/*
		* SyntaxError extends Exception
Expand DownExpand Up		@@ -2830,7 +2895,7 @@ static PyMemberDef SyntaxError_members[] = {

		ComplexExtendsException(PyExc_Exception, SyntaxError, SyntaxError,
		0, 0, SyntaxError_members, 0,
		SyntaxError_str, "Invalid syntax.");
		SyntaxError_str,0,"Invalid syntax.");


		/*
Expand DownExpand Up		@@ -2890,7 +2955,7 @@ KeyError_str(PyObject *op)
		}

		ComplexExtendsException(PyExc_LookupError, KeyError, BaseException,
		0, 0, 0, 0, KeyError_str, "Mapping key not found.");
		0, 0, 0, 0, KeyError_str,0,"Mapping key not found.");


		/*
Expand Down