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Context-aware return

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  Context-aware return Steven D'Aprano <steve@pearwood.info> - 2015-09-11 03:54 +1000
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:03 +0200
    Re: Context-aware return Ben Finney <ben+python@benfinney.id.au> - 2015-09-11 04:12 +1000
    Re: Context-aware return Ben Finney <ben+python@benfinney.id.au> - 2015-09-11 04:14 +1000
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:21 +0200
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:24 +0200
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:34 +0200
    Re: Context-aware return Chris Angelico <rosuav@gmail.com> - 2015-09-11 04:39 +1000
    Re: Context-aware return Laura Creighton <lac@openend.se> - 2015-09-10 20:40 +0200
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:45 +0200
    Re: Context-aware return "Sven R. Kunze" <srkunze@mail.de> - 2015-09-10 20:56 +0200
    Re: Context-aware return Paul Rubin <no.email@nospam.invalid> - 2015-09-10 12:19 -0700
    Re: Context-aware return Grant Edwards <invalid@invalid.invalid> - 2015-09-10 19:23 +0000
      Re: Context-aware return Akira Li <4kir4.1i@gmail.com> - 2015-09-10 23:15 +0300
        Re: Context-aware return Grant Edwards <invalid@invalid.invalid> - 2015-09-10 20:27 +0000
      Re: Context-aware return random832@fastmail.us - 2015-09-10 16:42 -0400
      Re: Context-aware return Rustom Mody <rustompmody@gmail.com> - 2015-09-11 08:55 -0700
      Re: Context-aware return Rustom Mody <rustompmody@gmail.com> - 2015-09-11 08:55 -0700
    Re: Context-aware return Denis McMahon <denismfmcmahon@gmail.com> - 2015-09-11 00:41 +0000
      Re: Context-aware return Ned Batchelder <ned@nedbatchelder.com> - 2015-09-12 16:27 -0700
        Re: Context-aware return Steven D'Aprano <steve@pearwood.info> - 2015-09-17 12:31 +1000
    Re: Context-aware return Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-12 03:11 +0100

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#96739

On Sun, 13 Sep 2015 09:27 am, Ned Batchelder wrote:

> On Thursday, September 10, 2015 at 8:44:01 PM UTC-4, Denis McMahon wrote:
>> On Fri, 11 Sep 2015 03:54:14 +1000, Steven D'Aprano wrote:
>> 
>> > If I did this thing, would people follow me down the street booing and
>> > jeering and throwing things at me?
>> 
>> Yes
>> 
>> >>> x = func()
>> >>> x
>> >>> func()
>> >>> print x == func()
>> >>> assert x == func()
>> 
>> Would you expect the last two calls to func() to return 999 or "Awesome"?
>> Why? What is the material difference if any between interpreter (a)
>> displaying the return value and (b) comparing the return value with
>> another value.
>> 
>> Debugging nightmare!
> 
> I'll add my voice to the rising chorus condemning the very notion
> of a function that behaves this way!
> 
> Then, I'll give you an implementation (Python 2):

Excellent! *cough* I mean, well, that's interesting. Of course no sane
developer would ever use such a thing.

Thanks to everyone who tried to discourage me from using this.




-- 
Steven

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#96380

On 10/09/2015 18:54, Steven D'Aprano wrote:
> I have a function which is intended for use at the interactive interpreter,
> but may sometimes be used non-interactively. I wish to change it's output
> depending on the context of how it is being called.
>
> If the function is being called as if it were a procedure or command, that
> is the return result is just ignored, I want to return one thing. But if it
> is being called where the return result goes somewhere, I want to return
> something else. Most importantly, I don't want to pass a flag to the
> function myself, I want the function to know its own context.
>
> I don't mind if it is CPython only, or if it is a bit expensive.
>
> E.g.
>
> def func():
>      do_stuff()
>      if procedure:  # FIXME what goes here???
>          return "Awesome"
>      else:
>          return 999
>
> Now I can do this:
>
> x = func()
> assert x == 999
>
> L = [1, 2, func(), 4]
> assert L[2] == 999
>
> func()
> # interactive interpreter prints "Awesome"
>
> Is such a thing possible, and if so, how would I do it?
>
> If I did this thing, would people follow me down the street booing and
> jeering and throwing things at me?
>

Not unless you were thrown in the Australian equivalent of Broadmoor 
first.  For those who don't know, Broadmoor is a famous place in the UK 
for the criminally insane.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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