[Python-Dev] PEP for allowing 'raise NewException from None'

[Ethan Furman]

PEP: XXX
Title: Interpreter support for concurrent programming
Version: $Revision$
Last-Modified: $Date$
Author: Ethan Furman <ethan at stoneleaf.us>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Jan-2012
Python-Version: 3.3
Post-History:


Abstract
========

One of the open issues from PEP 3134 is suppressing context:  currently 
there is no way to do it.  This PEP proposes one.

Motivation
==========

There are two basic ways to generate exceptions: 1) Python does it 
(buggy code, missing resources, ending loops, etc.); and, 2) manually 
(with a raise statement).

When writing libraries, or even just custom classes, it can become 
necessary to raise exceptions; moreover it can be useful, even 
necessary, to change from one exception to another.  To take an example 
from my dbf module:

     try:
         value = int(value)
     except Exception:
         raise DbfError(...)

Whatever the original exception was (ValueError, TypeError, or something 
   else) is irrelevant.  The exception from this point on is a DbfError, 
and the original exception is of no value.  However, if this exception 
is printed, we would currently see both.


Alternatives
============
Several possibilities have been put forth:

   - raise as NewException()

     Reuses the 'as' keyword; can be confusing since we are not really 
reraising the originating exception

   - raise NewException() from None

     Follows existing syntax of explicitly declaring the originating 
exception

   - exc = NewException(); exc.__context__ = None; raise exc

     Very verbose way of the previous method

   - raise NewException.no_context(...)

     Make context suppression a class method.

All of the above options will require changes to the core.


Proposal
========

I proprose going with the second option:

     raise NewException from None

It has the advantage of using the existing pattern of explicitly setting 
the cause:

     raise KeyError() from NameError()

but because the 'cause' is None the previous context is discarded. 
There is a patch to this effect attached to Issue6210 
(http://bugs.python.org/issue6210).


Copyright
=========

This document has been placed in the public domain.


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