[Python-Dev] PEP 415: Implementing PEP 409 differently (original) (raw)
Benjamin Peterson benjamin at python.org
Mon Feb 27 02:30:10 CET 2012
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PEP: 415 Title: Implementing PEP 409 differently Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Benjamin Peterson <benjamin at 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.
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