[Python-Dev] Assumed reflexivity of rich comparison operators (original) (raw)

Jared Flatow jflatow at northwestern.edu
Fri Jun 6 22:44:14 CEST 2008

On Jun 6, 2008, at 3:24 PM, Guido van Rossum wrote:

On Fri, Jun 6, 2008 at 1:10 PM, Jared Flatow <jflatow at northwestern.edu> wrote:

PEP 207 (http://www.python.org/dev/peps/pep-0207/) states in the fourth clause of the proposed resolutions to concerns:

"The reflexivity rules are assumed by Python. Thus, the interpreter may swap y>x with x<y, y>=x with x<=y, and may swap the arguments of x==y and x!=y." However, if this is the case, why does Python allow the definition of both pairs of le, ge and lt, gt for a single class, since users have no guarantee over which will be called? Currently, if I do not want x = y to mean the same thing as y <= x (and believe it or not I believe I have a good use case for doing this), I find it hard to believe that your users will be happy with that though. :-)

In this case, I am my users and I would be very happy with it (but I
won't try to justify it :).

there is no reliable way of doing this. Does it help if I tell you that for "x y" we always try x.binop(y) before trying y.reversebinop(x), except in the case where y is an instance of a subclass of the class of x?

Yes, actually that explains quite a bit, and now I see that is the
case I am dealing with. y is an instance of a subclass of x, but the
class of x is the one that defines both the binops. I suppose it is
too much to ask to only call the reverse_binop if the subclass
overrides it.

However, if the decision is to not allow users to do this at all using operators (and force them to create specially named methods), what is the point of allowing the definition of both? _The same reason we allow (require) you to define add and radd. It is quite possible that for any particular binary operation "x y", the class of x doesn't know how to implement it, and then the class of y is tried with the reversed operation.

It seems very confusing to me (and indeed I was at first very confused what was going on), to tempt users to be able to define both but give no promise that if they do, the appropriate one will be called. Does anyone have a good explanation for this? I have explained it as well as I can.

Thanks very much, at least that is enough information to work around
reliably.

jared

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