Fix ExceptionGroup repr changing when original exception sequence is mutated

CHANGES.rst

@@ -3,6 +3,12 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Fixed the ``repr()`` of exception groups being affected by mutation of the
+  original exception sequence after construction
+  (`#154 <https://github.com/agronholm/exceptiongroup/issues/154>`_)
+
 **1.3.1**
 
 - Fixed ``AttributeError: 'TracebackException' object has no attribute 'exceptions'``

src/exceptiongroup/_exceptions.py

@@ -275,7 +275,9 @@ def __str__(self) -> str:
         return f"{self.message} ({len(self._exceptions)} sub-exception{suffix})"
 
     def __repr__(self) -> str:
-        return f"{self.__class__.__name__}({self.args[0]!r}, {self.args[1]!r})"
+        return (
+            f"{self.__class__.__name__}({self.args[0]!r}, {list(self._exceptions)!r})"
+        )
 
 
 class ExceptionGroup(BaseExceptionGroup[_ExceptionT_co], Exception):

tests/test_exceptions.py

@@ -882,7 +882,13 @@ def test_exceptions_mutate_original_sequence():
 
     exceptions.append(KeyError("bar"))
     assert excgrp.exceptions is exc_tuple
-    assert repr(excgrp) == (
-        "BaseExceptionGroup('foo', [ValueError(1), KeyboardInterrupt(), "
-        "KeyError('bar')])"
-    )
+    if sys.version_info < (3, 11):
+        # On < 3.11, the backport is active and stores exceptions as a tuple,
+        # so repr reflects the original (unmutated) exceptions.
+        # On >= 3.11, native BaseExceptionGroup is used. Whether the repr
+        # shows original or mutated exceptions depends on the CPython version
+        # (cpython#141736 fixes it for 3.13.12+, but not yet in 3.14).
+        # We only assert the backport behavior since we control it.
+        assert repr(excgrp) == (
+            "BaseExceptionGroup('foo', [ValueError(1), KeyboardInterrupt()])"
+        )