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oct() and hex() need to use __index__ when available and then emit a
Py3K warning when they fall back on __oct__ and __hex__.

Shouldn't we leave alone oct() and hex() (there is another issue for
deprecation warning for __oct__ and __hex__), and let people use the
versions from future_builtins?

This is what I'd like to do: In builtin oct/hex check if the special
method is declared. If it is, Py3k warn and use it. If not, check if we
can do it the Py3k way (with PyNumber_ToBase). Sound good?

@Georg:
That is a possibility. Would need to add the proper Py3K warning to the 
current builtins, though.
@Benjamin:
You don't want to warn if it will work for Py3K, so don't warn if 
__index__ is used.

I meant not to. Sorry, that wasn't clear.

I don't assume there are too many people using oct() and hex() with
custom types anyway :)
Since PyNumber_ToBase and the new oct() format octal numbers with "0o"
instead of "0", I still think it's better if you need to import the new
oct() from future_builtins. For hex(), I have no preference.

Here's a patch for that. It warns for hex() when __hex__ is used,
otherwise it uses __index__. (This doesn't work for old style classes
because ob_tp->tp_as_number->tp_hex is filled.) Use of oct() just gets a
warning. The __hex__ and __oct__ methods of builtin number objects
should probably be removed.

The warning message for oct() should include that the output will be
different too. Else this is fine with me.

Done in r63425.

I have reverted this. See
mail.python.org/pipermail/python-checkins/2008-May/069843.html.

This was rejected by Guido.