[Python-Dev] Bug in PyNumber_InPlacePower implementation?

[Greg Ewing]

Guido:

> Me:
> >   a **= b
> > 
> > has the potential to in-place-modify b instead of a!
> 
> Not quite -- the arguments won't be reversed, but it may call B's
> inplace power function with an A instance as the first argument. 

Hmmm, I see. I guess it's just a matter of being aware
that this can happen and doing the necessary type tests.

> This can return NotImplemented if it doesn't know what to do 
> in that case.

Although if you're implementing inplace-power you're
probably also implementing non-inplace-power, in which case
the right thing is probably to call that instead. I
don't think that will happen automatically if you
return NotImplemented -- or will it?

> It's a pretty esoteric case ... The correct code would
> be pretty hairy I think (the non-inplace ternary is hairy enough).

I think the existing ternary_op routine could be converted
fairly easily into one that could be used for both. Just pass
in two slot arguments, and use one for the first arg and
the other one for the second and third args. For non-inplace
ops, call it with the non-inplace slot for both.

It's not a big deal, I suppose, but it would be nice if
it could be made consistent with the other in-place ops.

Greg Ewing, Computer Science Dept, +--------------------------------------+
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