PEP: 415 Title: Implement context suppression with exception attributes
Version: $Revision$ Last-Modified: $Date$ Author: Benjamin Peterson
<benjamin@python.org> BDFL-Delegate: Alyssa Coghlan Status: Final Type:
Standards Track Content-Type: text/x-rst Created: 26-Feb-2012
Python-Version: 3.3 Post-History: 26-Feb-2012 Replaces: 409 Resolution:
https://mail.python.org/pipermail/python-dev/2012-May/119467.html

Abstract

PEP 409 introduced support for the raise exc from None construct to
allow the display of the exception context to be explicitly suppressed.
This PEP retains the language level changes already implemented in PEP
409, but replaces the underlying implementation mechanism with a simpler
approach based on a new __suppress_context__ attribute on all
BaseException instances.

PEP Acceptance

This PEP was accepted by Alyssa Coghlan on the 14th of May, 2012.

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

Copyright

This document has been placed in the public domain.



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