Always create the exceptions tuple at init and return it from the exceptions property

Fixes #143.

Author: agronholm

CHANGES.rst

@@ -14,6 +14,10 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 - Changed ``BaseExceptionGroup.__init__()`` to directly call
   ``BaseException.__init__()`` instead of the superclass ``__init__()`` in order to
   emulate the CPython behavior (broken or not) (PR by @cfbolz)
+- Changed the ``exceptions`` attribute to always return the same tuple of exceptions,
+  created from the original exceptions sequence passed to ``BaseExceptionGroup`` to
+  match CPython behavior
+  (`#143 <https://github.com/agronholm/exceptiongroup/issues/143>`_)
 
 **1.2.2**
 

src/exceptiongroup/_exceptions.py

@@ -101,8 +101,7 @@ def __new__(
                         )
 
         instance = super().__new__(cls, __message, __exceptions)
-        instance._message = __message
-        instance._exceptions = __exceptions
+        instance._exceptions = tuple(__exceptions)
         return instance
 
     def __init__(
@@ -126,7 +125,7 @@ def add_note(self, note: str) -> None:
 
     @property
     def message(self) -> str:
-        return self._message
+        return self.args[0]
 
     @property
     def exceptions(
@@ -276,7 +275,7 @@ def __str__(self) -> str:
         return f"{self.message} ({len(self._exceptions)} sub-exception{suffix})"
 
     def __repr__(self) -> str:
-        return f"{self.__class__.__name__}({self.message!r}, {self._exceptions!r})"
+        return f"{self.__class__.__name__}({self.args[0]!r}, {self.args[1]!r})"
 
 
 class ExceptionGroup(BaseExceptionGroup[_ExceptionT_co], Exception):

tests/test_exceptions.py

@@ -864,3 +864,18 @@ class MyExceptionGroup(base, MyException):
         MyExceptionGroup(
             "...", [Exception()]
         )  # does not try to call MyException.__init__
+
+
+def test_exceptions_mutate_original_sequence():
+    exceptions = [ValueError(1), KeyboardInterrupt()]
+    excgrp = BaseExceptionGroup("foo", exceptions)
+    exc_tuple = excgrp.exceptions
+    assert isinstance(exc_tuple, tuple)
+    assert list(exc_tuple) == exceptions
+
+    exceptions.append(KeyError("bar"))
+    assert excgrp.exceptions is exc_tuple
+    assert repr(excgrp) == (
+        "BaseExceptionGroup('foo', [ValueError(1), KeyboardInterrupt(), "
+        "KeyError('bar')])"
+    )