pep for changing pep 409 implementation

pep-0415.txt

@@ -0,0 +1,78 @@
+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 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.
+
+
+..
+   Local Variables:
+   mode: indented-text
+   indent-tabs-mode: nil
+   sentence-end-double-space: t
+   fill-column: 70
+   coding: utf-8
+   End: