[3.14] gh-141732: Fix ExceptionGroup repr changing when original exception sequence is mutated (GH-141736)

Doc/library/exceptions.rst

@@ -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.

Include/cpython/pyerrors.h

@@ -18,6 +18,7 @@ typedef struct{
  PyException_HEAD
  PyObject *msg;
  PyObject *excs;
+ PyObject *excs_str;
 } PyBaseExceptionGroupObject;
 
 typedef struct{

Lib/test/test_exception_group.py

@@ -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
@@ -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".*MySeq\.__repr__\(\) must return a str, not NoneType"
+ ):
+ ExceptionGroup("test", seq)
+
+ seq = MySeq(ValueError)
+ with self.assertRaises(ValueError):
+ BaseExceptionGroup("test", seq)
+
+
 
 def create_simple_eg():
  excs = []

Misc/NEWS.d/next/Core_and_Builtins/2025-11-19-16-40-24.gh-issue-141732.PTetqp.rst

@@ -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.

Objects/exceptions.c

@@ -683,7 +683,7 @@
  0, 0, 0, 0, 0, \
  Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC, \
  PyDoc_STR(EXCDOC), EXCSTORE ## _traverse, \
  EXCSTORE ## _clear, 0, 0, 0, 0, 0, 0, 0, &_ ## EXCBASE, \
  0, 0, 0, offsetof(Py ## EXCSTORE ## Object, dict), \
  EXCSTORE ## _init, 0, 0, \
 };
@@ -695,12 +695,12 @@
 
 #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, \
@@ -793,7 +793,7 @@
 }
 
 ComplexExtendsException(PyExc_Exception, StopIteration, StopIteration,
- 0, 0, StopIteration_members, 0, 0,
+ 0, 0, StopIteration_members, 0, 0, 0,
  "Signal the end from iterator.__next__().");
 
 
@@ -866,7 +866,7 @@
 };
 
 ComplexExtendsException(PyExc_BaseException, SystemExit, SystemExit,
- 0, 0, SystemExit_members, 0, 0,
+ 0, 0, SystemExit_members, 0, 0, 0,
  "Request to exit from the interpreter.");
 
 /*
@@ -891,6 +891,7 @@
 
  PyObject *message = NULL;
  PyObject *exceptions = NULL;
+ PyObject *exceptions_str = NULL;
 
  if (!PyArg_ParseTuple(args,
  "UO:BaseExceptionGroup.__new__",
@@ -906,6 +907,18 @@
  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;
@@ -989,9 +1002,11 @@
 
  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;
 }
 
@@ -1030,6 +1045,7 @@
  PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
  Py_CLEAR(self->msg);
  Py_CLEAR(self->excs);
+ Py_CLEAR(self->excs_str);
  return BaseException_clear(op);
 }
 
@@ -1047,6 +1063,7 @@
  PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
  Py_VISIT(self->msg);
  Py_VISIT(self->excs);
+ Py_VISIT(self->excs_str);
  return BaseException_traverse(op, visit, arg);
 }
 
@@ -1064,6 +1081,54 @@
  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
@@ -1698,7 +1763,7 @@
 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.");
 
 /*
@@ -1807,7 +1872,7 @@
 }
 
 static PyObject *
 ImportError_str(PyObject *op)
 {
  PyImportErrorObject *self = PyImportErrorObject_CAST(op);
  if (self->msg && PyUnicode_CheckExact(self->msg)){
@@ -1864,7 +1929,7 @@
  return res;
 }
 
 static PyMemberDef ImportError_members[] ={
 {"msg", _Py_T_OBJECT, offsetof(PyImportErrorObject, msg), 0,
  PyDoc_STR("exception message")},
 {"name", _Py_T_OBJECT, offsetof(PyImportErrorObject, name), 0,
@@ -1876,17 +1941,17 @@
 {NULL} /* Sentinel */
 };
 
 static PyMethodDef ImportError_methods[] ={
 {"__reduce__", ImportError_reduce, METH_NOARGS},
 {NULL}
 };
 
 ComplexExtendsException(PyExc_Exception, ImportError,
  ImportError, 0 /* new */,
  ImportError_methods, ImportError_members,
  0 /* getset */, ImportError_str,
  "Import can't find module, or can't find name in "
  "module.");
 
 /*
  * ModuleNotFoundError extends ImportError
@@ -2356,7 +2421,7 @@
 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.");
 
 
@@ -2497,7 +2562,7 @@
 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
@@ -2631,7 +2696,7 @@
 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
@@ -2830,7 +2895,7 @@
 
 ComplexExtendsException(PyExc_Exception, SyntaxError, SyntaxError,
  0, 0, SyntaxError_members, 0,
- SyntaxError_str, "Invalid syntax.");
+ SyntaxError_str, 0, "Invalid syntax.");
 
 
 /*
@@ -2890,7 +2955,7 @@
 }
 
 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.");
 
 
 /*