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Date: Mon, 8 Dec 2014 11:34:23 +1100
Subject: Re: Python Iterables struggling using map() built-in
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Mon, Dec 8, 2014 at 11:12 AM, Roy Smith <roy@panix.com> wrote:
> Ugh.  When I see "while foo", my brain says, "OK, you're about to see a
> loop which is controlled by the value of foo being changed inside the
> loop".  That's not at all what's happening here, so my brain runs into a
> wall.

I agree, with the caveat that this kind of thing makes a fine infinite loop:

while "No exception raised":
    # do stuff that can raise an exception
while "Playing more games":
    play_game()
    if not still_playing: break
    reset_game_board()

Nobody expects a string literal to actually become false inside the
loop. With a local name, yes, I would expect it to at least have a
chance of becoming false.

> Next problem, what the heck is "res"?  We're not back in the punch-card
> days.  We don't have to abbreviate variable names to save columns.
> Variable names are supposed to describe what they hold, and thus help
> you understand the code.  I have no idea what "res" is supposed to be.
> Residue?  Result?  Rest_of_items?  Response?  None of these make much
> sense here, so I'm just left befuddled.

I take it as "result", which makes plenty of sense to me. It's the
thing that's about to be yielded. Given that there's not much else you
can say in a meta-function like zip(), I have no problem with that.
Here's a slightly different example:

def mark_last(it):
    it = iter(it)
    lastres = sentinel = object()
    while "more values coming":
        res = next(it, sentinel)
        if lastres is not sentinel: yield (lastres, res is sentinel)
        if res is sentinel: return
        lastres = res

Use of "res" for "result" and "lastres" to mean "res as of the
previous iteration of the loop" seems fine to me.

ChrisA