[Python-Dev] #2651 - KeyError does not round trip strings (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Wed Aug 4 23:57:07 CEST 2010

2010/8/5 Fred Drake <fdrake at acm.org>:

2010/8/4 Łukasz Langa <lukasz at langa.pl>:

1. The patch makes KeyError behave analogically to IOError so that the first arg is now a message and the second is the actual key. I agree with Antoine; there's no point to this.

2. Some people suggest adding e.key to KeyError. I like the idea but in my opinion currently it is not implementable in a reliable way. This is interesting and useful. I'd be really happy to see e.key be present if the key is known (because it was specifically provided to the constructor: KeyError(key=...)), or not present if the key isn't known.  (The idea is much less interesting if code can't distinguish between the key-is-known and the key-not-known cases.) The runtime and standard library should be adjusted to provide the key whenever possible, of course. Though I doubt this would break anything, we've lived without this long enough that the it doesn't represent a sufficient failing that the moratorium should be broken.  It can wait.

+1 on what Fred said (i.e. post-moratorium, add a keyword-only "key" argument to KeyError, set "e.key" only if that argument is supplied, update the standard library to supply it and use a default message of "'Key not found: %r' % key" if the key argument is supplied without an explicit message). Also +1 for doing the equivalent with AttributeError and an "attr" keyword only argument.

Since a keyword-only approach doesn't actually break any current code, I'm only -0 on doing that for 3.2 rather than -1.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

More information about the Python-Dev mailing list