[Python-Dev] Bad interaction of index and sequence repeat (original) (raw)

Guido van Rossum guido at python.org
Fri Jul 28 20:31:09 CEST 2006

On 7/28/06, Nick Coghlan <ncoghlan at gmail.com> wrote:

Nick Coghlan wrote: > Tim Peters wrote: >> OTOH, in the long discussion about PEP 357, I'm not sure anyone except >> Travis was clear on whether nbindex was meant to apply only to >> sequence /slicing/ or was meant to apply "everywhere an object gets >> used in an index-like context". Clipping makes sense only for the >> former, but it looks like the implementation treats it more like the >> latter. This was probably exacerbated by: >> >> http://mail.python.org/pipermail/python-dev/2006-February/060663.html >> >> [Travis] >> There are other places in Python that check specifically for int objects >> and long integer objects and fail with anything else. Perhaps all of >> these should aslo call the index slot. >> >> [Guido] >> Right, absolutely. >> >> This is a mess :-) > > I've been trawling through the code a bit, and I don't think it's as bad as > all that.

Damn, it really is a mess. . . nbindex returns the Pyssizet directly, and a whole heap of the code expects errors to be signalled via returning -1 before checking PyErrOccurred(). To get it to work without clipping everywhere, wraplenfunc (typeobject.c), PyEvalSliceIndex (ceval.c), PyNumberIndex (abstract.c) and sequencerepeat (abstract.c) all had to be modified to recognize PYSSIZETMIN and PYSSIZETMAX as potential error flags (in order to clear the overflow error for PyEvalSliceIndex, and in order to propagate the exception for the other three). And using this approach still means that (2**100).index() raises an OverflowError. It would probably be cleaner to change the signature of nbindex to return a PyObject *, and let the code that uses it worry about how (or even whether!) to convert PyLong results to a Pyssizet.

No time to look through the code here, but IMO it's acceptable (at least for 2.5) if (2100).index() raises OverflowError, as long as x[:2100] silently clips. index() is primarily meant to return a value useful for indexing concrete sequences, and 2**100 isn't.

Certainly the exception is preferrable to the silently truncated result currently returned.

Fortunately there's some extra time since we're now going to do a third beta.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

More information about the Python-Dev mailing list