PEP 678: more rejected ideas (GH-2205)

Author: Zac-HD

pep-0678.rst

@@ -15,13 +15,14 @@ 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, 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.
+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
+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 package, and common code patterns with retries.
+in the standard library, Hypothesis and ``cattrs`` packages, and common code
+patterns with retries.
 
 
 Motivation
@@ -32,17 +33,18 @@ 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 with retries may wish to note which iteration or timestamp raised which
-  error - especially if re-raising them in an ``ExceptionGroup``
+  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`` ).
+  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,
+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
+field ``__note__`` to ``BaseException``, which can be assigned a string - and
 if assigned, is automatically displayed in formatted tracebacks.
 
 
@@ -106,10 +108,21 @@ exception includes a note of the minimal failing example::
         +------------------------------------
 
 
+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
+``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.
 
@@ -118,7 +131,7 @@ 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__`` ,
+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``
@@ -128,11 +141,11 @@ 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
+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 [1]_.
+to breakage without warning [4]_.
 
-We are also unaware of any code which *would* be broken by adding ``__note__`` ;
+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.
 
@@ -142,23 +155,23 @@ 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" [2]_.
+and explained as part of the tutorial "Errors and Exceptions" [5]_.
 
 
 
 Reference Implementation
 ========================
 
-``BaseException.__note__`` was implemented in [3]_ and released in CPython 3.11.0a3,
-following discussions related to :pep:`654`. [4]_ [5]_ [6]_
+``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.)
----------------------------------------
+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
@@ -174,19 +187,19 @@ eliminates these problems.
 ---------------------------------------
 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.
+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 [7]_, but not for libraries which call user code.
+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__`` :
+exception chaining, and are unnecessary with ``BaseException.__note__``:
 
 .. code-block:: python
 
@@ -215,6 +228,9 @@ exception chaining, and are unnecessary with ``BaseException.__note__`` :
     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
 --------------------------------------------------
@@ -229,27 +245,84 @@ proposed ``__note__`` semantics, but this would be rarely and inconsistently
 applicable.
 
 
-Store notes in ``ExceptionGroup`` s
------------------------------------
+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.
+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://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers
-.. [2] https://github.com/python/cpython/pull/30158
-.. [3] https://github.com/python/cpython/pull/29880
-.. [4] https://discuss.python.org/t/accepting-pep-654-exception-groups-and-except/10813/9
-.. [5] https://github.com/python/cpython/pull/28569#discussion_r721768348
-.. [6] https://bugs.python.org/issue45607
-.. [7] https://github.com/python/cpython/blob/69ef1b59983065ddb0b712dac3b04107c5059735/Lib/http/client.py#L596-L597
+.. [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/30158
+.. [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