105 lines
3.0 KiB
Plaintext
105 lines
3.0 KiB
Plaintext
PEP: 415
|
||
Title: Implement context suppression with exception attributes
|
||
Version: $Revision$
|
||
Last-Modified: $Date$
|
||
Author: Benjamin Peterson <benjamin@python.org>
|
||
BDFL-Delegate: Nick 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 Nick 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.
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|