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

Nick Coghlan ncoghlan at gmail.com
Thu Aug 5 14:44:49 CEST 2010

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

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

Shall we do an e.index for IndexErrors as well? I don't recall stumbling over that need, but the parallel makes it tempting.  I expect is should be a separate patch, though. Antoine's right about using keyword args from C, though.  I'd expect a new helper function that handles this specific case, since it's likely to be common.  Whether it used a keyword are or just performed a setattr after the exception is created doesn't really matter.

Yeah, helper functions for C code that accepted the extra arguments would do the trick.

I actually had a look to see what IOError does with its attributes and I think it qualifies as being a little on the special side (replicated in both 2.6 and the py3k branch):

io1 = IOError(1) io2 = IOError(1, 2) io3 = IOError(1, 2, 3) io4 = IOError(1, 2, 3, 4) io1, io2, io3, io4 (IOError(1,), IOError(1, 2), IOError(1, 2), IOError(1, 2, 3, 4)) io1.errno, io2.errno, io3.errno, io4.errno (None, 1, 1, None) io1.strerror, io2.strerror, io3.strerror, io4.strerror (None, 2, 2, None) io1.filename, io2.filename, io3.filename, io4.filename (None, None, 3, None)

One argument = no attributes set Two arguments = errno, strerror set (including if second argument is explicitly None) Three arguments = errno, strerror, filename set Four or more arguments = no attributes set

Keyword arguments are not supported by IOError (or EnvironmentError, which is where the above behaviour is actually implemented).

That precedent would deem it acceptable to adopt a backwards compatible protocol that still allowed arbitrary positional arguments for Key/Attribute/IndexError, but treated the 2 argument case specially.

However, the IOError behaviour really doesn't strike me as a particularly good example for us to be following, so PEP 3151 may want to consider the issue of tidying up those exception signatures.

The "right" way to go still appears to be to allow arbitrary positional arguments (which go into .args) for backwards compatibility, then add appropriate keyword-only arguments. IOError could get some keyword only arguments as well, retaining the somewhat odd behaviour above for backwards compatibility reasons (although the repr in the 3 argument case should be fixed).

Cheers, Nick.

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

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