gh-141732: Fix `ExceptionGroup` repr changing when original exception sequence is mutated#141736

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Merged

iritkatriel merged 45 commits into python:main from dr-carlos:exception-group-repr

Dec 7, 2025

Doc/library/exceptions.rst

-Original file line number
+Diff line change
@@ Expand Up @@
  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 @@

Include/cpython/pyerrors.h

-Original file 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 @@

Lib/test/test_exception_group.py

-Original file 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".*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 = []
@@ Expand Down @@

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

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.

Objects/exceptions.c

-Original file line number
+Diff line change
@@ Expand Up / @@ -694,12 +694,12 @@ 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 @@
 }
 ComplexExtendsException(PyExc_Exception, StopIteration, StopIteration,
-, 0, StopIteration_members, 0, 0,
+, 0, StopIteration_members, 0, 0, 0,
  "Signal the end from iterator.__next__().");
@@ Expand DownExpand Up / @@ -865,7 +865,7 @@ static PyMemberDef SystemExit_members[] ={ @@
 };
 ComplexExtendsException(PyExc_BaseException, SystemExit, SystemExit,
-, 0, SystemExit_members, 0, 0,
+, 0, SystemExit_members, 0, 0, 0,
  "Request to exit from the interpreter.");
 /*
@@ Expand All @@
  PyObject *message = NULL;
  PyObject *exceptions = NULL;
+ PyObject *exceptions_str = NULL;
  if (!PyArg_ParseTuple(args,
  "UO:BaseExceptionGroup.__new__",
@@ Expand All @@
  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 @@
  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 / @@ -1029,6 +1044,7 @@ 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 @@
  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 / @@ -1063,6 +1080,54 @@ 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 / @@ -1697,7 +1762,7 @@ static PyMethodDef BaseExceptionGroup_methods[] ={ @@
 ComplexExtendsException(PyExc_BaseException, BaseExceptionGroup,
  BaseExceptionGroup, BaseExceptionGroup_new /* new */,
  BaseExceptionGroup_methods, BaseExceptionGroup_members,
-/* getset */, BaseExceptionGroup_str,
+/* getset */, BaseExceptionGroup_str, BaseExceptionGroup_repr,
  "A combination of multiple unrelated exceptions.");
 /*
@@ Expand DownExpand Up / @@ -2425,7 +2490,7 @@ 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 / @@ -2566,7 +2631,7 @@ static PyMethodDef NameError_methods[] ={ @@
 ComplexExtendsException(PyExc_Exception, NameError,
  NameError, 0,
  NameError_methods, NameError_members,
-, BaseException_str, "Name not found globally.");
+, BaseException_str, 0, "Name not found globally.");
 /*
  * UnboundLocalError extends NameError
@@ Expand DownExpand Up / @@ -2700,7 +2765,7 @@ static PyMethodDef AttributeError_methods[] ={ @@
 ComplexExtendsException(PyExc_Exception, AttributeError,
  AttributeError, 0,
  AttributeError_methods, AttributeError_members,
-, BaseException_str, "Attribute not found.");
+, BaseException_str, 0, "Attribute not found.");
 /*
  * SyntaxError extends Exception
@@ Expand DownExpand Up / @@ -2899,7 +2964,7 @@ static PyMemberDef SyntaxError_members[] ={ @@
 ComplexExtendsException(PyExc_Exception, SyntaxError, SyntaxError,
 , 0, SyntaxError_members, 0,
- SyntaxError_str, "Invalid syntax.");
+ SyntaxError_str, 0, "Invalid syntax.");
 /*
@@ Expand DownExpand Up / @@ -2959,7 +3024,7 @@ KeyError_str(PyObject *op) @@
 }
 ComplexExtendsException(PyExc_LookupError, KeyError, BaseException,
-, 0, 0, 0, KeyError_str, "Mapping key not found.");
+, 0, 0, 0, KeyError_str, 0, "Mapping key not found.");
 /*
@@ Expand Down @@

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gh-141732: Fix `ExceptionGroup` repr changing when original exception sequence is mutated #141736