386 lines
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ReStructuredText
386 lines
16 KiB
ReStructuredText
PEP: 678
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Title: Enriching Exceptions with Notes
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Author: Zac Hatfield-Dodds <zac@zhd.dev>
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Sponsor: Irit Katriel
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Discussions-To: https://discuss.python.org/t/pep-678-enriching-exceptions-with-notes/13374
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Status: Accepted
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Type: Standards Track
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Content-Type: text/x-rst
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Requires: 654
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Created: 20-Dec-2021
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Python-Version: 3.11
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Post-History: 27-Jan-2022
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Resolution: https://discuss.python.org/t/pep-678-enriching-exceptions-with-notes/13374/100
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Abstract
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========
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Exception objects are typically initialized with a message that describes the
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error which has occurred. Because further information may be available when
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the exception is caught and re-raised, or included in an ``ExceptionGroup``,
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this PEP proposes to add ``BaseException.add_note(note)``, a
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``.__notes__`` attribute holding a list of notes so added, and to
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update the builtin traceback formatting code to include notes in the formatted
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traceback following the exception string.
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This is particularly useful in relation to :pep:`654` ``ExceptionGroup``\ s,
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which make previous workarounds ineffective or confusing. Use cases have been
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identified in the standard library, Hypothesis and ``cattrs`` packages, and
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common code patterns with retries.
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Motivation
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==========
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When an exception is created in order to be raised, it is usually initialized
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with information that describes the error that has occurred. There are cases
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where it is useful to add information after the exception was caught. For
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example,
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- testing libraries may wish to show the values involved in a failing
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assertion, or the steps to reproduce a failure (e.g. ``pytest`` and
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``hypothesis``; example below).
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- code which retries an operation on error may wish to associate an iteration,
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timestamp, or other explanation with each of several errors - especially if
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re-raising them in an ``ExceptionGroup``.
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- programming environments for novices can provide more detailed descriptions
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of various errors, and tips for resolving them.
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Existing approaches must pass this additional information around while keeping
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it in sync with the state of raised, and potentially caught or chained,
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exceptions. This is already error-prone, and made more difficult by :pep:`654`
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``ExceptionGroup``\ s, so the time is right for a built-in solution. We
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therefore propose to add:
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- a new method ``BaseException.add_note(note: str)``,
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- ``BaseException.__notes__``, a list of note strings added using
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``.add_note()``, and
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- support in the builtin traceback formatting code such that notes are
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displayed in the formatted traceback following the exception string.
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Example usage
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-------------
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::
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>>> try:
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... raise TypeError('bad type')
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... except Exception as e:
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... e.add_note('Add some information')
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... raise
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...
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Traceback (most recent call last):
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File "<stdin>", line 2, in <module>
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TypeError: bad type
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Add some information
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>>>
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When collecting exceptions into an exception group, we may want to add context
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information for the individual errors. In the following example with
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`Hypothesis' proposed support for ExceptionGroup
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<https://github.com/HypothesisWorks/hypothesis/pull/3191>`__, each exception
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includes a note of the minimal failing example::
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from hypothesis import given, strategies as st, target
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@given(st.integers())
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def test(x):
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assert x < 0
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assert x > 0
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+ Exception Group Traceback (most recent call last):
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| File "test.py", line 4, in test
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| def test(x):
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| File "hypothesis/core.py", line 1202, in wrapped_test
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| raise the_error_hypothesis_found
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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| ExceptionGroup: Hypothesis found 2 distinct failures.
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+-+---------------- 1 ----------------
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| Traceback (most recent call last):
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| File "test.py", line 6, in test
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| assert x > 0
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| ^^^^^^^^^^^^
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| AssertionError: assert -1 > 0
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| Falsifying example: test(
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| x=-1,
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| )
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+---------------- 2 ----------------
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| Traceback (most recent call last):
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| File "test.py", line 5, in test
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| assert x < 0
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| ^^^^^^^^^^^^
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| AssertionError: assert 0 < 0
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| Falsifying example: test(
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| x=0,
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| )
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+------------------------------------
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Non-goals
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---------
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Tracking multiple notes as a list, rather than by concatenating strings when
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notes are added, is intended to maintain the distinction between the
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individual notes. This might be required in specialized use cases, such
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as translation of the notes by packages like ``friendly-traceback``.
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However, ``__notes__`` is *not* intended to carry structured data. If your
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note is for use by a program rather than display to a human, `we recommend
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<https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/26>`__
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instead (or additionally) choosing a convention for an attribute, e.g.
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``err._parse_errors = ...`` on the error or ``ExceptionGroup``.
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As a rule of thumb, we suggest that you should prefer `exception chaining
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<https://docs.python.org/3/tutorial/errors.html#exception-chaining>`__ when the
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error is going to be re-raised or handled as an individual error, and prefer
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``.add_note()`` when you want to avoid changing the exception type or
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are collecting multiple exception objects to handle together. [1]_
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Specification
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=============
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``BaseException`` gains a new method ``.add_note(note: str)``. If ``note`` is
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a string, ``.add_note(note)`` appends it to the ``__notes__`` list, creating
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the attribute if it does not already exist. If ``note`` is not a string,
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``.add_note()`` raises ``TypeError``.
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Libraries may clear existing notes by modifying or deleting the ``__notes__``
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list, if it has been created, including clearing all notes with
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``del err.__notes__``. This allows full control over the attached notes,
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without overly complicating the API or adding multiple names to
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``BaseException.__dict__``.
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When an exception is displayed by the interpreter's builtin traceback-rendering code,
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its notes (if there are any) appear immediately after the exception message, in the order
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in which they were added, with each note starting on a new line.
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If ``__notes__`` has been created, ``BaseExceptionGroup.subgroup`` and
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``BaseExceptionGroup.split`` create a new list for each new instance, containing
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the same contents as the original exception group's ``__notes__``.
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We *do not* specify the expected behaviour when users have assigned a non-list
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value to ``__notes__``, or a list which contains non-string elements.
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Implementations might choose to emit warnings, discard or ignore bad values,
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convert them to strings, raise an exception, or do something else entirely.
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Backwards Compatibility
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=======================
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System-defined or "dunder" names (following the pattern ``__*__``) are part of
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the language specification, with `unassigned names reserved for future use and
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subject to breakage without warning
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<https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers>`__.
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We are also unaware of any code which *would* be broken by adding ``__notes__``.
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We were also unable to find any code which would be broken by the addition of
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``BaseException.add_note()``: while searching Google and `GitHub finds several
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definitions <https://grep.app/search?q=.add_note%28&filter[lang][0]=Python>`__
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of an ``.add_note()`` method, none of them are on a subclass of
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``BaseException``.
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How to Teach This
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=================
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The ``add_note()`` method and ``__notes__`` attribute will be documented as
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part of the language standard, and explained as part of `the "Errors and
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Exceptions" tutorial <https://github.com/python/cpython/pull/30441>`__.
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Reference Implementation
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========================
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Following discussions related to :pep:`654` [2]_, an early version of this
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proposal was `implemented in <https://github.com/python/cpython/pull/29880>`__
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and released in CPython 3.11.0a3, with a mutable string-or-none ``__note__``
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attribute.
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`CPython PR #31317 <https://github.com/python/cpython/pull/31317>`__
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implements ``.add_note()`` and ``__notes__``.
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Rejected Ideas
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==============
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.. _print_idea:
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Use ``print()`` (or ``logging``, etc.)
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--------------------------------------
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Reporting explanatory or contextual information about an error by printing or
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logging has historically been an acceptable workaround. However, we dislike
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the way this separates the content from the exception object it refers to -
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which can lead to "orphan" reports if the error was caught and handled later,
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or merely significant difficulties working out which explanation corresponds to
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which error. The new ``ExceptionGroup`` type intensifies these existing
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challenges.
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Keeping the ``__notes__`` attached to the exception object, in the same way as
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the ``__traceback__`` attribute, eliminates these problems.
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``raise Wrapper(explanation) from err``
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---------------------------------------
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An alternative pattern is to use exception chaining: by raising a 'wrapper'
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exception containing the context or explanation ``from`` the current exception,
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we avoid the separation challenges from ``print()``. However, this has two key
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problems.
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First, it changes the type of the exception, which is often a breaking change
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for downstream code. We consider *always* raising a ``Wrapper`` exception
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unacceptably inelegant; but because custom exception types might have any
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number of required arguments we can't always create an instance of the *same*
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type with our explanation. In cases where the exact exception type is known
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this can work, such as the standard library ``http.client`` `code
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<https://github.com/python/cpython/blob/69ef1b59983065ddb0b712dac3b04107c5059735/Lib/http/client.py#L596-L597>`__,
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but not for libraries which call user code.
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Second, exception chaining reports several lines of additional detail, which
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are distracting for experienced users and can be very confusing for beginners.
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For example, six of the eleven lines reported for this simple example relate to
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exception chaining, and are unnecessary with ``BaseException.add_note()``:
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.. code-block:: python
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class Explanation(Exception):
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def __str__(self):
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return "\n" + str(self.args[0])
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try:
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raise AssertionError("Failed!")
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except Exception as e:
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raise Explanation("You can reproduce this error by ...") from e
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.. code-block::
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$ python example.py
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Traceback (most recent call last):
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File "example.py", line 6, in <module>
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raise AssertionError(why)
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AssertionError: Failed!
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# These lines are
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The above exception was the direct cause of ... # confusing for new
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# users, and they
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Traceback (most recent call last): # only exist due
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File "example.py", line 8, in <module> # to implementation
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raise Explanation(msg) from e # constraints :-(
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Explanation: # Hence this PEP!
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You can reproduce this error by ...
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**In cases where these two problems do not apply, we encourage use of exception
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chaining rather than** ``__notes__``.
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An assignable ``__note__`` attribute
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------------------------------------
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The first draft and implementation of this PEP defined a single attribute
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``__note__``, which defaulted to ``None`` but could have a string assigned.
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This is substantially simpler if, and only if, there is at most one note.
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To promote interoperability and support translation of error messages by
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libraries such as ``friendly-traceback``, without resorting to dubious parsing
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heuristics, we therefore settled on the ``.add_note()``-and-``__notes__`` API.
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Subclass Exception and add note support downstream
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--------------------------------------------------
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Traceback printing is built into the C code, and reimplemented in pure Python
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in ``traceback.py``. To get ``err.__notes__`` printed from a downstream
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implementation would *also* require writing custom traceback-printing code;
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while this could be shared between projects and reuse some pieces of
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traceback.py [3]_ we prefer to implement this once, upstream.
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Custom exception types could implement their ``__str__`` method to include our
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proposed ``__notes__`` semantics, but this would be rarely and inconsistently
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applicable.
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Don't attach notes to ``Exception``\ s, just store them in ``ExceptionGroup``\ s
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--------------------------------------------------------------------------------
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The initial motivation for this PEP was to associate a note with each error
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in an ``ExceptionGroup``. At the cost of a remarkably awkward API and the
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cross-referencing problem discussed `above <print_idea>`__, this
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use-case could be supported by storing notes on the ``ExceptionGroup``
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instance instead of on each exception it contains.
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We believe that the cleaner interface, and other use-cases described above,
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are sufficient to justify the more general feature proposed by this PEP.
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Add a helper function ``contextlib.add_exc_note()``
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---------------------------------------------------
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It `was suggested
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<https://www.reddit.com/r/Python/comments/rmrvxv/pep_678_enriching_exceptions_with_notes/hptbul1/>`__
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that we add a utility such as the one below to the standard library. We do not
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see this idea as core to the proposal of this PEP, and thus leave it for later
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or downstream implementation - perhaps based on this example code:
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.. code-block:: python
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@contextlib.contextmanager
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def add_exc_note(note: str):
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try:
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yield
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except Exception as err:
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err.add_note(note)
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raise
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with add_exc_note(f"While attempting to frobnicate {item=}"):
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frobnicate_or_raise(item)
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Augment the ``raise`` statement
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-------------------------------
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One discussion proposed ``raise Exception() with "note contents"``, but this
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does not address the original motivation of compatibility with
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``ExceptionGroup``.
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Furthermore, we do not believe that the problem we are solving requires or
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justifies new language syntax.
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Acknowledgements
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================
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We wish to thank the many people who have assisted us through conversation,
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code review, design advice, and implementation: Adam Turner, Alex Grönholm,
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André Roberge, Barry Warsaw, Brett Cannon, CAM Gerlach, Carol Willing, Damian,
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Erlend Aasland, Etienne Pot, Gregory Smith, Guido van Rossum, Irit Katriel,
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Jelle Zijlstra, Ken Jin, Kumar Aditya, Mark Shannon, Matti Picus, Petr
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Viktorin, Will McGugan, and pseudonymous commenters on Discord and Reddit.
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References
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==========
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.. [1] this principle was established in the 2003 mail thread which led to :pep:`3134`,
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and included a proposal for a group-of-exceptions type!
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https://mail.python.org/pipermail/python-dev/2003-January/032492.html
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.. [2] particularly those at https://bugs.python.org/issue45607,
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https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/9,
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https://github.com/python/cpython/pull/28569#discussion_r721768348, and
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.. [3] We note that the ``exceptiongroup`` backport package maintains an exception
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hook and monkeypatch for ``TracebackException`` for Pythons older than 3.11,
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and encourage library authors to avoid creating additional and incompatible
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backports. We also reiterate our preference for builtin support which
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makes such measures unnecessary.
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Copyright
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=========
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This document is placed in the public domain or under the
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CC0-1.0-Universal license, whichever is more permissive.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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