[issue12323] ElementPath 1.3 expressions

[Stefan Behnel]

Stefan Behnel added the comment:

I agree that [0] should be treated as a visible error as it's easy to get wrong. It's certainly too late to change this to 0-based indexing and I think it's ok to keep it 1-based for XPath compatibility (or at least similarity) as that's what people will expect it to mimic. I came up with a similar fix for lxml, BTW.

Regarding the behaviour of negative indices, it's clearly broken and unexpected and "-1" is definitely not a valid XML tag name (as which it's currently interpreted). That's a change for Py3.4+ only though. Either disable negative indices completely and raise an exception at parse time or read "-X" as "last()-X". I would also be ok with the latter as it feels natural to support this in a Python context. But that's a new feature then, not a bug fix. And the fact that "last()-X" already exists tends to speak against it. It's more of an "if the intention is clear, why raise an exception" kind of thing.

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nosy: +scoder

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<http://bugs.python.org/issue12323>
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