PEP: 409
Title: Suppressing exception context
Version: $Revision$
Last-Modified: $Date$
Author: Ethan Furman <ethan@stoneleaf.us>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Jan-2012
Python-Version: 3.3
Post-History: 2012-01-27


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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