8000 [3.11] gh-99619: fix error in documentation of ExceptionGroup.derive() (GH-99621) by miss-islington · Pull Request #99720 · python/cpython · GitHub
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[3.11] gh-99619: fix error in documentation of ExceptionGroup.derive() (GH-99621) #99720

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Nov 23, 2022
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[3.11] gh-99619: fix error in documentation of ExceptionGroup.derive() (GH-99621) #99720

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33 changes: 27 additions & 6 deletions Doc/library/exceptions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -934,21 +934,42 @@ their subgroups based on the types of the contained exceptions.

.. method:: derive(excs)

Returns an exception group with the same :attr:`message`,
:attr:`__traceback__`, :attr:`__cause__`, :attr:`__context__`
and :attr:`__notes__` but which wraps the exceptions in ``excs``.
Returns an exception group with the same :attr:`message`, but which
wraps the exceptions in ``excs``.

This method is used by :meth:`subgroup` and :meth:`split`. A
subclass needs to override it in order to make :meth:`subgroup`
and :meth:`split` return instances of the subclass rather
than :exc:`ExceptionGroup`. ::
than :exc:`ExceptionGroup`.

:meth:`subgroup` and :meth:`split` copy the :attr:`__traceback__`,
:attr:`__cause__`, :attr:`__context__` and :attr:`__notes__` fields from
the original exception group to the one returned by :meth:`derive`, so
these fields do not need to be updated by :meth:`derive`. ::

>>> class MyGroup(ExceptionGroup):
... def derive(self, exc):
... return MyGroup(self.message, exc)
...
>>> MyGroup("eg", [ValueError(1), TypeError(2)]).split(TypeError)
(MyGroup('eg', [TypeError(2)]), MyGroup('eg', [ValueError(1)]))
>>> e = MyGroup("eg", [ValueError(1), TypeError(2)])
>>> e.add_note("a note")
>>> e.__context__ = Exception("context")
>>> e.__cause__ = Exception("cause")
>>> try:
... raise e
... except Exception as e:
... exc = e
...
>>> match, rest = exc.split(ValueError)
>>> exc, exc.__context__, exc.__cause__, exc.__notes__
(MyGroup('eg', [ValueError(1), TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
>>> match, match.__context__, match.__cause__, match.__notes__
(MyGroup('eg', [ValueError(1)]), Exception('context'), Exception('cause'), ['a note'])
>>> rest, rest.__context__, rest.__cause__, rest.__notes__
(MyGroup('eg', [TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
>>> exc.__traceback__ is match.__traceback__ is rest.__traceback__
True


Note that :exc:`BaseExceptionGroup` defines :meth:`__new__`, so
subclasses that need a different constructor signature need to
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