344 lines
13 KiB
ReStructuredText
344 lines
13 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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Status: Draft
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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:
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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 the
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exception is caught and re-raised, or included in an ``ExceptionGroup``, this PEP
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proposes to add a ``.__note__`` attribute and update the builtin traceback formatting
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code to include it in the formatted traceback following the exception string.
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This is particularly useful in relation to :pep:`654` ``ExceptionGroup``\ s, which
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make previous workarounds ineffective or confusing. Use cases have been identified
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in the standard library, Hypothesis and ``cattrs`` packages, and common code
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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.
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For example,
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- testing libraries may wish to show the values involved in a failing assertion,
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or the steps to reproduce a failure (e.g. ``pytest`` and ``hypothesis``; example below).
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- code which retries an operation on error may wish to associate an iteration, timestamp,
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or other explanation with each of several errors - especially if re-raising them in
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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 (e.g. ``friendly-traceback``).
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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, exceptions.
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This is already error-prone, and made more difficult by :pep:`654` ``ExceptionGroup``\ s,
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so the time is right for a built-in solution. We therefore propose to add a mutable
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field ``__note__`` to ``BaseException``, which can be assigned a string - and
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if assigned, is automatically displayed in formatted tracebacks.
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Example usage
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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.__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
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to add context information for the individual errors. In the following
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example with `Hypothesis' proposed support for ExceptionGroup
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<https://github.com/HypothesisWorks/hypothesis/pull/3191>`__, each
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exception 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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``__note__`` is *not* intended to carry structured data. If your note is for use by
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a program rather than display to a human, we recommend instead choosing a convention
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for an attribute like e.g. ``err._parse_errors = ...`` on the error or ``ExceptionGroup`` [1]_ [2]_
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As a rule of thumb, prefer `exception chaining <https://docs.python.org/3/tutorial/errors.html#exception-chaining>`__
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when the error is going to be re-raised or handled as an individual error, and prefer
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``__note__`` when you are collecting multiple exception objects to handle together or later. [3]_
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Specification
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=============
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``BaseException`` gains a new mutable attribute ``__note__``, which defaults to
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``None`` and may have a string assigned. When an exception with a note is displayed,
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the note is displayed immediately after the exception.
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Assigning a new string value overrides an existing note; if concatenation is desired
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users are responsible for implementing it with e.g.::
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e.__note__ = msg if e.__note__ is None else e.__note__ + "\n" + msg
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It is an error to assign a non-string-or-``None`` value to ``__note__``,
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or to attempt to delete the attribute.
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``BaseExceptionGroup.subgroup`` and ``BaseExceptionGroup.split``
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copy the ``__note__`` of the original exception group to the parts.
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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 the
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language specification, with unassigned names reserved for future use and subject
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to breakage without warning [4]_.
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We are also unaware of any code which *would* be broken by adding ``__note__``;
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assigning to a ``.__note__`` attribute already *works* on current versions of
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Python - the note just won't be displayed with the traceback and exception message.
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How to Teach This
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=================
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The ``__note__`` attribute will be documented as part of the language standard,
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and explained as part of the tutorial "Errors and Exceptions" [5]_.
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Reference Implementation
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========================
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``BaseException.__note__`` was implemented in [6]_ and released in CPython 3.11.0a3,
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following discussions related to :pep:`654`. [7]_ [8]_ [9]_
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Rejected Ideas
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==============
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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 logging
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has historically been an acceptable workaround. However, we dislike the way this
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separates the content from the exception object it refers to - which can lead to
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"orphan" reports if the error was caught and handled later, or merely significant
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difficulties working out which explanation corresponds to which error.
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The new ``ExceptionGroup`` type intensifies these existing challenges.
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Keeping the ``__note__`` attached to the exception object, like the traceback,
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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' exception
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containing the context or explanation ``from`` the current exception, we avoid the
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separation challenges from ``print()``. However, this has two key problems.
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First, it changes the type of the exception, which is often a breaking change for
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downstream code. We consider *always* raising a ``Wrapper`` exception unacceptably
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inelegant; but because custom exception types might have any number of required
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arguments we can't always create an instance of the *same* type with our explanation.
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In cases where the exact exception type is known this can work, such as the standard
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library ``http.client`` code [10]_, but not for libraries which call user code.
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Second, exception chaining reports several lines of additional detail, which are
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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.__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)
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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 the following exception: # 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
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of exception chaining rather than** ``__note__``.
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Subclass Exception and add ``__note__`` downstream
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--------------------------------------------------
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Traceback printing is built into the C code, and reimplemented in pure Python in
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traceback.py. To get ``err.__note__`` printed from a downstream implementation
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would *also* require writing custom traceback-printing code; while this could
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be shared between projects and reuse some pieces of traceback.py we prefer to
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implement this once, upstream.
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Custom exception types could implement their ``__str__`` method to include our
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proposed ``__note__`` semantics, but this would be rarely and inconsistently
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applicable.
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Store notes in ``ExceptionGroup``\ s
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------------------------------------
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Initial discussions proposed making a more focussed change by thinking about how to
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associate messages with the nested exceptions in ``ExceptionGroup`` s, such as a list
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of notes or mapping of exceptions to notes. However, this would force a remarkably
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awkward API and retains a lesser form of the cross-referencing problem discussed
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under "use ``print()``" above; if this PEP is rejected we prefer the status quo.
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Finally, of course, ``__note__`` is not only useful with ``ExceptionGroup`` s!
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Possible Future Enhancements
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============================
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In addition to rejected alternatives, there have been a range of suggestions which
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we believe should be deferred to a future version, when we have more experience with
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the uses (and perhaps misuses) of ``__note__``.
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Allow any object, and cast to string for display
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------------------------------------------------
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We have not identified any scenario where libraries would want to do anything but either
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concatenate or replace notes, and so the additional complexity and interoperability
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challenges do not seem justified.
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Permitting any object would also force any future structured API to change the behaviour
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of already-legal code, whereas expanding the permitted contents of ``__note__`` from strings
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to include other objects is fully backwards-compatible. In the absence of any proposed
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use-case (see also `Non-goals`_), we prefer to begin with a restrictive API that can
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be relaxed later.
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Add a helper function ``contextlib.add_exc_note()``
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---------------------------------------------------
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It was suggested that we add a utility such as the one below to the standard
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library. We are open to this idea, but do not see it as a core part of the
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proposal of this PEP as it can be added as an enhancement later.
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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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if err.__note__ is None:
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err.__note__ = note
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else:
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err.__note__ = err.__note__ + "\n\n" + 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 ``ExceptionGroup``.
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Furthermore, we do not believe that the problem we are solving requires or justifies
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new language syntax.
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References
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==========
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.. [1] https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/26
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.. [2] https://bugs.python.org/issue46431
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.. [3] 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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.. [4] https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers
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.. [5] https://github.com/python/cpython/pull/30441
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.. [6] https://github.com/python/cpython/pull/29880
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.. [7] https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/9
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.. [8] https://github.com/python/cpython/pull/28569#discussion_r721768348
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.. [9] https://bugs.python.org/issue45607
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.. [10] https://github.com/python/cpython/blob/69ef1b59983065ddb0b712dac3b04107c5059735/Lib/http/client.py#L596-L597
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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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