python-peps/peps/pep-0415.rst

PEP: 415
Title: Implement context suppression with exception attributes
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin@python.org>
BDFL-Delegate: Alyssa Coghlan
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Feb-2012
Python-Version: 3.3
Post-History: 26-Feb-2012
Replaces: 409
Resolution: https://mail.python.org/pipermail/python-dev/2012-May/119467.html


Abstract
========

:pep:`409` introduced support for the ``raise exc from None`` construct to
allow the display of the exception context to be explicitly suppressed.
This PEP retains the language level changes already implemented in :pep:`409`,
but replaces the underlying implementation mechanism with a simpler approach
based on a new ``__suppress_context__`` attribute on all ``BaseException``
instances.


PEP Acceptance
==============

This PEP was accepted by Alyssa Coghlan on the 14th of May, 2012.


Rationale
=========

:pep:`409` changes ``__cause__`` to be ``Ellipsis`` by default. Then if
``__cause__`` is set to ``None`` by ``raise exc from None``, no context or cause
will be printed should the exception be uncaught.

The main problem with this scheme is it complicates the role of
``__cause__``. ``__cause__`` should indicate the cause of the exception not
whether ``__context__`` should be printed or not. This use of ``__cause__`` is
also not easily extended in the future. For example, we may someday want to
allow the programmer to select which of ``__context__`` and ``__cause__`` will
be printed. The :pep:`409` implementation is not amenable to this.

The use of ``Ellipsis`` is a hack. Before :pep:`409`, ``Ellipsis`` was used
exclusively in extended slicing. Extended slicing has nothing to do with
exceptions, so it's not clear to someone inspecting an exception object why
``__cause__`` should be set to ``Ellipsis``. Using ``Ellipsis`` by default for
``__cause__`` makes it asymmetrical with ``__context__``.


Proposal
========

A new attribute on ``BaseException``, ``__suppress_context__``, will
be introduced. Whenever ``__cause__`` is set, ``__suppress_context__``
will be set to ``True``. In particular, ``raise exc from cause``
syntax will set ``exc.__suppress_context__`` to ``True``. Exception
printing code will check for that attribute to determine whether
context and cause will be printed. ``__cause__`` will return to its
original purpose and values.

There is precedence for ``__suppress_context__`` with the
``print_line_and_file`` exception attribute.

To summarize, ``raise exc from cause`` will be equivalent to::

    exc.__cause__ = cause
    raise exc

where ``exc.__cause__ = cause`` implicitly sets
``exc.__suppress_context__``.


Patches
=======

There is a patch on `Issue 14133`_.


References
==========

.. _issue 14133:
   http://bugs.python.org/issue14133


Copyright
=========

This document has been placed in the public domain.


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-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								PEP: 415
-												more descriptive title

											
										
										
											2012-05-15 00:02:57 -04:00
+								Title: Implement context suppression with exception attributes
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Version: $Revision$
 								Last-Modified: $Date$
 								Author: Benjamin Peterson <benjamin@python.org>
-												Update references to Alyssa Coghlan (#3475)

* For most cases (including all PEP metadata), replace birth name with chosen name
* Birth name has been retained in parentheses when this makes other references
  substantially clearer (e.g. references to mailing list posts, elected members
  of the inaugural Steering Council)
											
										
										
											2023-10-11 08:05:51 -04:00
+								BDFL-Delegate: Alyssa Coghlan
-now implemented

											
										
										
											2012-05-15 01:10:26 -04:00
+								Status: Final
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Type: Standards Track
 								Content-Type: text/x-rst
 								Created: 26-Feb-2012
-												Add the BDFL-Delegate field, using myself as the test case.

Including the delegate's email address would be nice, but the PEP writer lives in docutils upstream rather than our PEPs repo and I'm not updating docutils just to mask an additional field, nor am I inclined to figure out how to move the writer definition downstream where it belongs

											
										
										
											2012-05-06 02:31:52 -04:00
+								Python-Version: 3.3
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Post-History: 26-Feb-2012
-												Make the relationship between 409 and 415 clearer

											
										
										
											2012-05-15 07:53:25 -04:00
+								Replaces: 409
-												promote m.p.o links to https

											
										
										
											2017-06-11 15:02:39 -04:00
+								Resolution: https://mail.python.org/pipermail/python-dev/2012-May/119467.html
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
 								Abstract
 								========
-												Several PEPs: Use explicit `:pep:` and `:rfc:` roles (#2209)


											
										
										
											2022-01-21 06:03:51 -05:00
+								:pep:`409` introduced support for the ``raise exc from None`` construct to
-												Make the relationship between 409 and 415 clearer

											
										
										
											2012-05-15 07:53:25 -04:00
+								allow the display of the exception context to be explicitly suppressed.
-												Several PEPs: Use explicit `:pep:` and `:rfc:` roles (#2209)


											
										
										
											2022-01-21 06:03:51 -05:00
+								This PEP retains the language level changes already implemented in :pep:`409`,
-												Make the relationship between 409 and 415 clearer

											
										
										
											2012-05-15 07:53:25 -04:00
+								but replaces the underlying implementation mechanism with a simpler approach
 								based on a new ``__suppress_context__`` attribute on all ``BaseException``
 								instances.
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
-												Accept PEP 415

											
										
										
											2012-05-14 08:08:40 -04:00
 								PEP Acceptance
 								==============
-												Update references to Alyssa Coghlan (#3475)

* For most cases (including all PEP metadata), replace birth name with chosen name
* Birth name has been retained in parentheses when this makes other references
  substantially clearer (e.g. references to mailing list posts, elected members
  of the inaugural Steering Council)
											
										
										
											2023-10-11 08:05:51 -04:00
+								This PEP was accepted by Alyssa Coghlan on the 14th of May, 2012.
-												Accept PEP 415

											
										
										
											2012-05-14 08:08:40 -04:00
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Rationale
 								=========
-												Several PEPs: Use explicit `:pep:` and `:rfc:` roles (#2209)


											
										
										
											2022-01-21 06:03:51 -05:00
+								:pep:`409` changes ``__cause__`` to be ``Ellipsis`` by default. Then if
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								``__cause__`` is set to ``None`` by ``raise exc from None``, no context or cause
 								will be printed should the exception be uncaught.
 								The main problem with this scheme is it complicates the role of
 								``__cause__``. ``__cause__`` should indicate the cause of the exception not
 								whether ``__context__`` should be printed or not. This use of ``__cause__`` is
 								also not easily extended in the future. For example, we may someday want to
 								allow the programmer to select which of ``__context__`` and ``__cause__`` will
-												Several PEPs: Use explicit `:pep:` and `:rfc:` roles (#2209)


											
										
										
											2022-01-21 06:03:51 -05:00
+								be printed. The :pep:`409` implementation is not amenable to this.
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
-												Several PEPs: Use explicit `:pep:` and `:rfc:` roles (#2209)


											
										
										
											2022-01-21 06:03:51 -05:00
+								The use of ``Ellipsis`` is a hack. Before :pep:`409`, ``Ellipsis`` was used
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								exclusively in extended slicing. Extended slicing has nothing to do with
 								exceptions, so it's not clear to someone inspecting an exception object why
 								``__cause__`` should be set to ``Ellipsis``. Using ``Ellipsis`` by default for
 								``__cause__`` makes it asymmetrical with ``__context__``.
-												Accept PEP 415

											
										
										
											2012-05-14 08:08:40 -04:00
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Proposal
 								========
-												I understand what Nick meant now

											
										
										
											2012-02-27 18:59:27 -05:00
+								A new attribute on ``BaseException``, ``__suppress_context__``, will
-												implicitly set __suppress_context__ when __cause__ is set

											
										
										
											2012-03-05 16:43:34 -05:00
+								be introduced. Whenever ``__cause__`` is set, ``__suppress_context__``
 								will be set to ``True``. In particular, ``raise exc from cause``
 								syntax will set ``exc.__suppress_context__`` to ``True``. Exception
 								printing code will check for that attribute to determine whether
 								context and cause will be printed. ``__cause__`` will return to its
 								original purpose and values.
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
 								There is precedence for ``__suppress_context__`` with the
 								``print_line_and_file`` exception attribute.
-												give equivalent code for raise from

											
										
										
											2012-02-27 10:51:11 -05:00
+								To summarize, ``raise exc from cause`` will be equivalent to::
 								    exc.__cause__ = cause
 								    raise exc
-												implicitly set __suppress_context__ when __cause__ is set

											
										
										
											2012-03-05 16:43:34 -05:00
+								where ``exc.__cause__ = cause`` implicitly sets
 								``exc.__suppress_context__``.
-												Accept PEP 415

											
										
										
											2012-05-14 08:08:40 -04:00
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Patches
 								=======
 								There is a patch on `Issue 14133`_.
 								References
 								==========
 								.. _issue 14133:
-												fix link

											
										
										
											2012-04-29 21:27:35 -04:00
+								   http://bugs.python.org/issue14133
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
-												Accept PEP 415

											
										
										
											2012-05-14 08:08:40 -04:00
-												pep for changing pep 409 implementation

											
										
										
											2012-02-26 20:29:14 -05:00
+								Copyright
 								=========
 								This document has been placed in the public domain.
 								..
 								   Local Variables:
 								   mode: indented-text
 								   indent-tabs-mode: nil
 								   sentence-end-double-space: t
 								   fill-column: 70
 								   coding: utf-8
 								   End: