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PEP 678: Clarify how the notes tuple is updated and when it is copied #2377

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JelleZijlstra merged 6 commits into from
Mar 3, 2022
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PEP 678: Clarify how the notes tuple is updated and when it is copied #2377

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PEP 678: clarify internals and heading
Zac-HD Mar 2, 2022
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PEP 678: omit impl details
Zac-HD Mar 2, 2022
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PEP 678: clarify rejected idea
Zac-HD Mar 2, 2022
2491278
PEP 678: link cross-reference
Zac-HD Mar 2, 2022
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PEP 678: final details update
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27 changes: 14 additions & 13 deletions pep-0678.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -140,10 +140,10 @@ are collecting multiple exception objects to handle together. [1]_
Specification
=============

``BaseException`` gains a new read-only attribute ``__notes__``, an initially
empty tuple, and a new method ``.add_note(note: str)``. ``note`` is added to
the exception's notes, which appear in the standard traceback after the
exception string. A ``TypeError`` is raised if ``note`` is not a string.
``BaseException`` gains a new new method ``.add_note(note: str)``. Notes are
exposed as a tuple via the read-only attribute ``__notes__``, and appear in
the standard traceback after the exception string. ``.add_note()`` raises
``TypeError`` if ``note`` is not a string.
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``del err.__notes__`` clears the contents of the ``__notes__`` attribute,
leaving it an empty tuple as if ``.add_note()`` had never been called. This
Expand Down Expand Up @@ -319,15 +319,16 @@ proposed ``__notes__`` semantics, but this would be rarely and inconsistently
applicable.


Store notes in ``ExceptionGroup``\ s
------------------------------------
Initial discussions proposed making a more focussed change by thinking about
how to associate messages with the nested exceptions in ``ExceptionGroup`` s,
such as a list of notes or mapping of exceptions to notes. However, this would
force a remarkably awkward API and retains a lesser form of the
cross-referencing problem discussed under "use ``print()``" above; if this PEP
is rejected we prefer the status quo. Finally, of course, ``__notes__`` are
not only useful with ``ExceptionGroup``\ s!
Don't attach notes to ``Exception``\ s, just store them in ``ExceptionGroup``\ s
--------------------------------------------------------------------------------
The initial motivation for this PEP was to associate a note with each error
in an ``ExceptionGroup``. At the cost of a remarkably awkward API and the
cross-referencing problem discussed under "use ``print()``" above, this
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use-case could be supported by storing notes on the ``ExceptionGroup``
instance instead of on each exception it contains.

We believe that the cleaner interface, and other use-cases described above,
are sufficient to justify the more general feature proposed by this PEP.



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