PEP: 415
Title: Implementing PEP 409 differently
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin@python.org>
BDFL-Delegate: Nick Coghlan
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Feb-2012
Python-Version: 3.3
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. 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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