[Tutor] diferent between " "(empty string) and None as condition

[boB Stepp]

On Sat, Apr 25, 2020 at 2:38 AM Cameron Simpson <cs at cskk.id.au> wrote:
>
> On 24Apr2020 23:07, boB Stepp <robertvstepp at gmail.com> wrote:
> >On Fri, Apr 24, 2020 at 7:56 AM Mats Wichmann <mats at wichmann.us> wrote:
> >
> >> === extra credit, no need to go here if you don't want:
> >>
> >> There are also cases where things are coded in such a way that None is a
> >> valid value, and can't be used as the sentinel, and you need yet another
> >> value to distinguish that something out-of-band happened...
> >
> >Would you please show a realistic example where one might do this --
> >where None can't be used as a sentinel because it is otherwise
> >occupied?
>
> Anything where a valid function result might be None.

Ah.  Just this one sentence made the light bulb glow brightly!

> Invented example:
>
>     from queue import Queue
>
>     EOF = object()
>     Q = Queue()
>     d = {'a': 0, 'b': 2}
>     for k in 'a', 'b', 'c':
>         Q.put( d.get(k) )
>     Q.put(EOF)
>     while True:
>         v = Q.get()
>         if v is EOF:
>             break
>         print("v =", v)
>
> Here we have a Queue onto which we might want to put _any_ value,
> including None. The consumer wnats to know when all the values have been
> received. For this purpose we make a special object instance EOF for the
> sentinel, which we know will not be used as a value.
>
> This kind of "make a special instance of object" is in fact a common
> idiom for sentinel in Python.

Thanks, Cameron.  The example ties it all together for me.

OT:  Cameron, just for your posts to Tutor where I am being responded
to, I find Gmail sending all of your posts to spam.  I don't know much
about the inner workings of email, but I notice in the details Gmail
provides:

from:Cameron Simpson <cs at cskk.id.au> reply-to:tutor at python.org,
boB Stepp <robertvstepp at gmail.com>,
tutor <tutor at python.org>
to:boB Stepp <robertvstepp at gmail.com>

The Tutor list is duplicated.  Would this be the cause of straight to
spam?  If yes, is this something correctable on your end?  Not that I
want to impose on your valuable time!

-- 
boB