[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

Tim Peters tim.peters at gmail.com
Sat Jun 30 12:37:53 EDT 2018

[Nick Coghlan]

> > ...

> > "NAME := EXPR" exists on a different level of complexity, since it

> > adds name binding in arbitrary expressions for the sake of minor

> > performance improvement in code written by developers that are

> > exceptionally averse to the use of vertical screen real estate,
> ...

Note that PEP 572 doesn't contain a single word about "performance"
(neither that specific word nor any synonym), and I gave only one thought
to it when writing Appendix A:  "is this going to slow anything down
significantly?".  The answer was never "yes", which I thought was
self-evident, so I never mentioned it.  Neither did Chris or Guido.

Best I can recall, nobody has argued for it on the grounds of
"performance". except in the indirect sense that sometimes it allows a more
compact way of reusing an expensive subexpression by giving it a name.
 Which they already do by giving it a name in a separate statement, so the
possible improvement would be in brevity rather than performance.

The attractions are instead in the areas of reducing redundancy, improving
clarity, allowing to remove semantically pointless indentation levels in
some cases, indeed trading away some horizontal whitespace in otherwise
nearly empty lines for freeing up a bit of vertical screen space, and in
the case of comprehensions/genexps adding straightforward ways to
accomplish some conceptually trivial things that at best require trickery
now (like emulating a cell object by hand).

Calling all that "for the sake of minor performance improvements" - which
isn't even in the list -  is sooooo far off base it should have left me
speechless - but it didn't ;-)

But now that you mention it, ya, there will be a trivial performance
improvement in some cases.  I couldn't care less about that, and can
confidently channel that Guido doesn't either.  It would remain fine by me
if assignment expressions ran trivially slower.
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