[Python-Dev] Comparison speed

[Guido van Rossum]

> [Martin v. Loewis]
> > Is this really exactly what Python would guarantee? I'm surprised that
> > x==x would always be true, but x!=x might be true also. In a type where
> > x!=x holds, wouldn't people also want to say that x==x might fail? IOW,
> > I had expected that you'd reduced it to
> >
> >   if (v == w && op == Py_EQ) /* then return Py_True */
> >   if (v == w && op == Py_NE) /* then return Py_False */

[Tim]
> I agree that would be more analogous to what PyObject_Compare() does.
> 
> I'm not sure either make sense for rich comparisons; for example, under
> IEEE-754 rules, a NaN must compare not-equal to everything, including
> itself(!), and richcmps are the only hope Python users have of modeling that.
> Doing those pointer checks before giving richcmps a chance would kill that
> hope.  Can we agree to drop this one until somebody produces stats saying
> it's important?  I have no reason to suspect that it is.

PEP 207 is quite explicit that == and != are not to be assumed each
other's complement.  It is silent on the x==x issue but the PEP
mentions IEEE 754 so I agree that this also shouldn't be cut short.

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