python-peps/pep-0415.txt

86 lines
2.4 KiB
Plaintext
Raw Blame History

This file contains invisible Unicode characters

This file contains invisible Unicode characters that are indistinguishable to humans but may be processed differently by a computer. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

PEP: 415
Title: Implementing PEP 409 differently
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Feb-2012
Post-History: 26-Feb-2012
Abstract
========
PEP 409 allows PEP 3134 exception contexts and causes to be suppressed when the
exception is printed. This is done using the ``raise exc from None``
syntax. This PEP proposes to implement context and cause suppression
differently.
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. The ``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
exc.__suppress_context__ = True
raise exc
Patches
=======
There is a patch on `Issue 14133`_.
References
==========
.. _issue 14133:
http://bugs.python.org/issue6210
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: