python-peps/pep-0678.rst

PEP: 678
Title: Enriching Exceptions with Notes
Author: Zac Hatfield-Dodds <zac@zhd.dev>
Sponsor: Irit Katriel
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Requires: 654
Created: 20-Dec-2021
Python-Version: 3.11
Post-History:


Abstract
========
Exception objects are typically initialized with a message that describes the
error which has occurred.  Because further information may be available when the
exception is caught and re-raised, or included in an ``ExceptionGroup``, this PEP
proposes to add a ``.__note__`` attribute and update the builtin traceback formatting
code to include it in the formatted traceback following the exception string.

This is particularly useful in relation to :pep:`654` ``ExceptionGroup``\ s, which
make previous workarounds ineffective or confusing.  Use cases have been identified
in the standard library, Hypothesis and ``cattrs`` packages, and common code
patterns with retries.


Motivation
==========
When an exception is created in order to be raised, it is usually initialized
with information that describes the error that has occurred. There are cases
where it is useful to add information after the exception was caught.
For example,

- testing libraries may wish to show the values involved in a failing assertion,
  or the steps to reproduce a failure (e.g. ``pytest`` and ``hypothesis``; example below).
- code which retries an operation on error may wish to associate an iteration, timestamp,
  or other explanation with each of several errors - especially if re-raising them in
  an ``ExceptionGroup``.
- programming environments for novices can provide more detailed descriptions
  of various errors, and tips for resolving them (e.g. ``friendly-traceback``).

Existing approaches must pass this additional information around while keeping
it in sync with the state of raised, and potentially caught or chained, exceptions.
This is already error-prone, and made more difficult by :pep:`654` ``ExceptionGroup``\ s,
so the time is right for a built-in solution.  We therefore propose to add a mutable
field ``__note__`` to ``BaseException``, which can be assigned a string - and
if assigned, is automatically displayed in formatted tracebacks.


Example usage
-------------

   >>> try:
   ...     raise TypeError('bad type')
   ... except Exception as e:
   ...     e.__note__ = 'Add some information'
   ...     raise
   ...
   Traceback (most recent call last):
     File "<stdin>", line 2, in <module>
   TypeError: bad type
   Add some information
   >>>

When collecting exceptions into an exception group, we may want
to add context information for the individual errors. In the following
example with `Hypothesis' proposed support for ExceptionGroup
<https://github.com/HypothesisWorks/hypothesis/pull/3191>`__, each
exception includes a note of the minimal failing example::

    from hypothesis import given, strategies as st, target

    @given(st.integers())
    def test(x):
        assert x < 0
        assert x > 0


    + Exception Group Traceback (most recent call last):
    |   File "test.py", line 4, in test
    |     def test(x):
    |
    |   File "hypothesis/core.py", line 1202, in wrapped_test
    |     raise the_error_hypothesis_found
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | ExceptionGroup: Hypothesis found 2 distinct failures.
    +-+---------------- 1 ----------------
        | Traceback (most recent call last):
        |   File "test.py", line 6, in test
        |     assert x > 0
        |     ^^^^^^^^^^^^
        | AssertionError: assert -1 > 0
        |
        | Falsifying example: test(
        |     x=-1,
        | )
        +---------------- 2 ----------------
        | Traceback (most recent call last):
        |   File "test.py", line 5, in test
        |     assert x < 0
        |     ^^^^^^^^^^^^
        | AssertionError: assert 0 < 0
        |
        | Falsifying example: test(
        |     x=0,
        | )
        +------------------------------------


Non-goals
---------
``__note__`` is *not* intended to carry structured data.  If your note is for use by
a program rather than display to a human, we recommend instead choosing a convention
for an attribute like e.g. ``err._parse_errors = ...`` on the error or ``ExceptionGroup`` [1]_ [2]_

As a rule of thumb, prefer `exception chaining <https://docs.python.org/3/tutorial/errors.html#exception-chaining>`__
when the error is going to be re-raised or handled as an individual error, and prefer
``__note__`` when you are collecting multiple exception objects to handle together or later. [3]_


Specification
=============

``BaseException`` gains a new mutable attribute ``__note__``, which defaults to
``None`` and may have a string assigned.  When an exception with a note is displayed,
the note is displayed immediately after the exception.

Assigning a new string value overrides an existing note; if concatenation is desired
users are responsible for implementing it with e.g.::

    e.__note__ = msg if e.__note__ is None else e.__note__ + "\n" + msg

It is an error to assign a non-string-or-``None`` value to ``__note__``,
or to attempt to delete the attribute.

``BaseExceptionGroup.subgroup`` and ``BaseExceptionGroup.split``
copy the ``__note__`` of the original exception group to the parts.


Backwards Compatibility
=======================

System-defined or "dunder" names (following the pattern ``__*__``) are part of the
language specification, with unassigned names reserved for future use and subject
to breakage without warning [4]_.

We are also unaware of any code which *would* be broken by adding ``__note__``;
assigning to a ``.__note__`` attribute already *works* on current versions of
Python - the note just won't be displayed with the traceback and exception message.



How to Teach This
=================

The ``__note__`` attribute will be documented as part of the language standard,
and explained as part of the tutorial "Errors and Exceptions" [5]_.



Reference Implementation
========================

``BaseException.__note__`` was implemented in [6]_ and released in CPython 3.11.0a3,
following discussions related to :pep:`654`. [7]_ [8]_ [9]_



Rejected Ideas
==============

Use ``print()`` (or ``logging``, etc.)
--------------------------------------
Reporting explanatory or contextual information about an error by printing or logging
has historically been an acceptable workaround.  However, we dislike the way this
separates the content from the exception object it refers to - which can lead to
"orphan" reports if the error was caught and handled later, or merely significant
difficulties working out which explanation corresponds to which error.
The new ``ExceptionGroup`` type intensifies these existing challenges.

Keeping the ``__note__`` attached to the exception object, like the traceback,
eliminates these problems.


``raise Wrapper(explanation) from err``
---------------------------------------
An alternative pattern is to use exception chaining: by raising a 'wrapper' exception
containing the context or explanation ``from`` the current exception, we avoid the
separation challenges from ``print()``.  However, this has two key problems.

First, it changes the type of the exception, which is often a breaking change for
downstream code.  We consider *always* raising a ``Wrapper`` exception unacceptably
inelegant; but because custom exception types might have any number of required
arguments we can't always create an instance of the *same* type with our explanation.
In cases where the exact exception type is known this can work, such as the standard
library ``http.client`` code [10]_, but not for libraries which call user code.

Second, exception chaining reports several lines of additional detail, which are
distracting for experienced users and can be very confusing for beginners.
For example, six of the eleven lines reported for this simple example relate to
exception chaining, and are unnecessary with ``BaseException.__note__``:

.. code-block:: python

    class Explanation(Exception):
        def __str__(self):
            return "\n" + str(self)

    try:
        raise AssertionError("Failed!")
    except Exception as e:
        raise Explanation("You can reproduce this error by ...") from e

.. code-block::

    $ python example.py
    Traceback (most recent call last):
    File "example.py", line 6, in <module>
        raise AssertionError(why)
    AssertionError: Failed!
                                                                            # These lines are
    The above exception was the direct cause of the following exception:    # confusing for new
                                                                            # users, and they
    Traceback (most recent call last):                                      # only exist due
    File "example.py", line 8, in <module>                                  # to implementation
        raise Explanation(msg) from e                                       # constraints :-(
    Explanation:                                                            # Hence this PEP!
    You can reproduce this error by ...

**In cases where these two problems do not apply, we encourage use
of exception chaining rather than** ``__note__``.


Subclass Exception and add ``__note__`` downstream
--------------------------------------------------
Traceback printing is built into the C code, and reimplemented in pure Python in
traceback.py. To get ``err.__note__`` printed from a downstream implementation
would *also* require writing custom traceback-printing code; while this could
be shared between projects and reuse some pieces of traceback.py we prefer to
implement this once, upstream.

Custom exception types could implement their ``__str__`` method to include our
proposed ``__note__`` 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, ``__note__`` is not only useful with ``ExceptionGroup`` s!



Possible Future Enhancements
============================

In addition to rejected alternatives, there have been a range of suggestions which
we believe should be deferred to a future version, when we have more experience with
the uses (and perhaps misuses) of ``__note__``.


Allow any object, and cast to string for display
------------------------------------------------
We have not identified any scenario where libraries would want to do anything but either
concatenate or replace notes, and so the additional complexity and interoperability
challenges do not seem justified.

Permitting any object would also force any future structured API to change the behaviour
of already-legal code, whereas expanding the permitted contents of ``__note__`` from strings
to include other objects is fully backwards-compatible.  In the absence of any proposed
use-case (see also `Non-goals`_), we prefer to begin with a restrictive API that can
be relaxed later.


Add a helper function ``contextlib.add_exc_note()``
---------------------------------------------------
It was suggested that we add a utility such as the one below to the standard
library. We are open to this idea, but do not see it as a core part of the
proposal of this PEP as it can be added as an enhancement later.

.. code-block:: python

    @contextlib.contextmanager
    def add_exc_note(note: str):
        try:
            yield
        except Exception as err:
            if err.__note__ is None:
                err.__note__ = note
            else:
                err.__note__ = err.__note__ + "\n\n" + note
            raise

    with add_exc_note(f"While attempting to frobnicate {item=}"):
        frobnicate_or_raise(item)


Augment the ``raise`` statement
-------------------------------
One discussion proposed ``raise Exception() with "note contents"``, but this
does not address the original motivation of compatibility with ``ExceptionGroup``.

Furthermore, we do not believe that the problem we are solving requires or justifies
new language syntax.

References
==========

.. [1] https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/26
.. [2] https://bugs.python.org/issue46431
.. [3] this principle was established in the 2003 mail thread which led to :pep:`3134`,
       and included a proposal for a group-of-exceptions type!
       https://mail.python.org/pipermail/python-dev/2003-January/032492.html
.. [4] https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers
.. [5] https://github.com/python/cpython/pull/30441
.. [6] https://github.com/python/cpython/pull/29880
.. [7] https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/9
.. [8] https://github.com/python/cpython/pull/28569#discussion_r721768348
.. [9] https://bugs.python.org/issue45607
.. [10] https://github.com/python/cpython/blob/69ef1b59983065ddb0b712dac3b04107c5059735/Lib/http/client.py#L596-L597



Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


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