[Python-Dev] Symmetric vs asymmetric symbols (was PEP 572: Do we really need a ":" in ":="?)

Chris Angelico rosuav at gmail.com
Fri Jul 6 00:02:59 EDT 2018

On Fri, Jul 6, 2018 at 12:48 PM, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> Python really has a strong C legacy and this is the area where I agree that
> C designers made a mistake by picking a symmetric symbol (=) for an
> asymmetric operation. On top of that, they picked an asymmetric digraph (!=)
> for a symmetric operation as well and Python (unfortunately) followed the
> crowd and ditched a much better alternative (<>).  My only hope is that
> Python 4.0 will allow ← to be used in place of either = or :=. :-)

Interesting. Looking over Python's binary operators, we have:

|, ^, &, +, *: symmetric (on ints)
-, /, //, **: asymmetric
<, >: mirrored operations
<=, >=: mirrored operations but not reflected
<<, >>: non-mirrored asymmetric
and, or: technically asymmetric but often treated as symmetric
in, not in: asymmetric
is, is not: symmetric

Which ones ought to have symmetric symbols, in an ideal world? Should
<= and >= be proper mirrors of each other? Are << and >> confusing? Is
it a problem that the ** operator is most decidedly asymmetric?

Personally, I'm very happy that the operators use the same symbols
that they do in other languages - U+002B PLUS SIGN means addition, for
instance - and everything else is secondary. But maybe this is one of
those "hidden elegances" that you're generally not *consciously* aware
of, but which makes things "feel right", like how Disney's "Moana" has
freedom to the right of the screen and duty to the left. Are there
languages where symmetric operations are always represented with
symmetric symbols and vice versa?


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