Help with sets

[Steve Howell]

On Oct 13, 7:25 pm, Lawrence D'Oliveiro <l... at geek-
central.gen.new_zealand> wrote:
> In message
> <aa6eafa0-7075-424c-abef-79cbc0dd3... at w19g2000yqb.googlegroups.com>, Steve
>
> Howell wrote:
> > I guess a lot depends on how you define "symmetry."  Is your
> > definition of "symmetry" equivalent to your definition of
> > "orthogonality"?
>
> No idea. It’s just that the example being discussed in this thread seemed to
> come under the old term “orthogonality”, so I wondered why a different term
> was being used.
>
> So far no-one has come up with a good excuse for using a different term.

Ask the authors of PEP 234 why they use the term symmetry:

http://www.python.org/dev/peps/pep-0234/

That was the original context of my comment.  The term "symmetry" gets
used a couple times in that PEP, and I think we're in violent
agreement that the concept of "symmetry" is wishy-washy at best.

Here is just one example from the PEP:

      The symmetry between "if x in y" and "for x in y"
      suggests that it should iterate over keys.  This symmetry has
been
      observed by many independently and has even been used to
"explain"
      one using the other.

I think I'm just making the same observation as you coming from a
different angle.  Why talk about "symmetry" when it's such a tricky
balance?