PEP-654: add BaseExceptionGroup//ExceptionGroup split

pep-0654.rst

@@ -126,14 +126,24 @@ Specification
 ExceptionGroup
 --------------
 
-The new builtin exception type, ``ExceptionGroup`` is a subclass of
-``BaseException``, so it is assignable to ``Exception.__cause__`` and
-``Exception.__context__``, and can be raised and handled as any exception
-with ``raise ExceptionGroup(...)`` and ``try: ... except ExceptionGroup: ...``.
-
-Its constructor takes two positional-only parameters: a message string and a
-sequence of the nested exceptions, for example:
+We propose to add two new builtin exception types: ``ExceptionGroup`` and
+``BaseExceptionGroup``. The former is a subclass of ``Exception`` and
+the latter is a subclass of ``BaseException``. Both are assignable to
+``Exception.__cause__`` and ``Exception.__context__``, and they can be
+raised and handled as any exception with ``raise ExceptionGroup(...)`` and
+``try: ... except ExceptionGroup: ...`` or ``raise BaseExceptionGroup(...)``
+and ``try: ... except BaseExceptionGroup: ...``.
+
+Both have a constructor that takes two positional-only parameters: a message
+string and a sequence of the nested exceptions, for example:
 ``ExceptionGroup('issues', [ValueError('bad value'), TypeError('bad type')])``.
+The difference between them is that ``ExceptionGroup`` can only wrap
+``Exception`` subclasses while ``BaseExceptionGroup`` can wrap any
+``BaseException`` subclass. A factory method that inspects the nested
+execptions and selects between ``ExceptionGroup`` and ``BaseExceptionGroup``
+makes the choice automatic. In the rest of the document, unless stated
+otherwise, when we refer to ``ExceptionGroup`` we mean either an
+``ExceptionGroup`` or a ``BaseExceptionGroup``.
 
 The ``ExceptionGroup`` class exposes these parameters in the fields ``message``
 and ``errors``.  A nested exception can also be an ``ExceptionGroup`` so the
@@ -944,17 +954,20 @@ propose in this PEP will not break any existing code:
   Programs will only be impacted by the changes proposed in this PEP once they
   begin to use ``ExceptionGroups`` and ``except*``.
 
+* An important concern was that ``except Exception:`` will continue to catch
+  almost all exceptions, and by making ``ExceptionGroup`` extend ``Exception``
+  we ensured that this will be the case.  ``BaseExceptionGroups`` will not be
+  caught, which is appropriate because they include exceptions that would not
+  have been caught by ``except Exception``.
 
 Once programs begin to use these features, there will be migration issues to
 consider:
 
-* An ``except Exception:`` clause will not catch ``ExceptionGroup`` because it
-  is derived from ``BaseException``. Any such clause will need to be replaced
-  by ``except (Exception, ExceptionGroup):`` or ``except *Exception:``.
-
-* Similarly, any ``except T:`` clause that wraps code which is now potentially
-  raising ``ExceptionGroup`` needs to become ``except *T:``, and its body may
-  need to be updated.
+* An ``except T:`` clause that wraps code which is now potentially raising
+  ``ExceptionGroup`` may need to become ``except *T:``, and its body may
+  need to be updated. This means that raising an ``ExceptionGroup`` is an
+  API-breaking change and will likely be done in new APIs rather than
+  added to existing ones.
 
 * Libraries that need to support older Python versions will not be able to use
   ``except*`` or raise ``ExceptionGroups``.