PEP: 415 Title: Implementing PEP 409 differently Version: $Revision$ Last-Modified: $Date$ Author: Benjamin Peterson <benja...@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 amendable 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 None`` syntax will cause ``exc.__suppress_context__`` to be set to ``True``. Exception printing code will check for the 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. 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. _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
