For the record, I am -1 for this change after discussion on #python-dev. There are three major arguments against the proposed approach:

1. Moratorium. If we don't stick to the rules set by ourselves, nobody will take us seriously. No exceptions, this has to wait.

2. Backwards incompatibility. The current implementation is backwards incompatible and doesn't provide a reliable way of returning `e.key`. There was an alternative discussed about including key= and message= keyword arguments to the exception. This would require writing an additional helper routine in C for dealing with keyword arguments and is thus a bit of a broader problem.

3. PEP 3151. While still in draft form, this PEP prepares a solution for more generic and consistent behaviour of exceptions. It mentions IOError and OSError at the moment but Alexander is right: if the PEP 3151 approach chooses a new form of argument handling for exceptions, lookup errors should try to conform with that.

For the above reasons, I would resolve this issue as 'after moratorium' and prepare a superseder that gathers all PEP 3151 related issues in the tracker.