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

Jared Flatow jflatow at northwestern.edu
Fri Jun 6 22:10:58 CEST 2008

Hi all,

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), there is no
reliable way of doing this. 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? 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?

Thanks! jared

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