Re: [PATCH] (Tiny) Tweak Python session null return value

[John Kitchin <jkitchin@andrew.cmu.edu>]

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I can see why you would want to see True/False there, but to get the value,
you need to specifically return what you want because AFAIK the body is
wrapped in a function that is evaluated to get the value, it is not simply
the last thing that gets evaluated. Your example clarified to me at least
why it would be tricky to figure it out, you can't rely on the last line,
for example. I don't know if there is some special Python variable that
contains that.

It is also a little strange to me to put return specifically in like this:

#+begin_src python :session :results value
  if some_condition:
      a = True
  else:
      a = False
return a
#+end_src

or:

#+BEGIN_SRC python :results value
a = 1
return True if a else False
#+END_SRC

#+RESULTS:
: True

because it is not valid syntax in a script (try it with :results output),
but this is how it has been for ob-python all along when what you want is
the value returned.

John

-----------------------------------
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu



On Mon, Feb 17, 2020 at 2:06 PM Jack Kamm <jackkamm@gmail.com> wrote:

> Hi John,
>
> John Kitchin <jkitchin@andrew.cmu.edu> writes:
>
> > I think None is correct. If you don't specify a return value in Python,
> > then a function returns None. I would expect that to happen in a Python
> > block too.
>
> Hmm, OK, thanks for your intuition, it's useful feedback.
>
> Working this out loud, I was considering the following sort of block:
>
> #+begin_src python :session :results value
>   if some_condition:
>       True
>   else:
>       False
> #+end_src
>
> #+RESULTS:
> : None
>
> Ideally, it would return True/False, but the current implementation
> cannot grab that result unfortunately. (In its defense, it at least
> doesn't crash like it did before).
>
> I was thinking not printing anything at all under "#+RESULTS" would be
> less surprising than printing "None". But both are admittedly surprising
> at first.
>
> I agree your preference of returning "None" is the more technically
> consistent behavior though.
>
> I don't want to rush a decision now, I think I need to gather more user
> feedback after the 9.4 release. I'll ask Bastien to revert that commit
> and put this off to 9.5, rather than writing an updated NEWS entry like
> he asked.
>
> Either way, we'll need to update Worg to clearly document what's going
> on here.
>

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