ob-ipython[1] provides a working alternative:

#+BEGIN_SRC jupyter-python :session :results output
  foo = 0
  for _ in range(10):
      foo += 1

      foo += 1

  print(foo)
#+END_SRC

#+RESULTS:
: 20

I've long wished that more org people would show ob-ipython some love.
Letting jupyter handle things on the backend seems like it should
simplifly things considerably.

[1] https://github.com/gregsexton/ob-ipython

Best,
Ista

On Tue, Nov 21, 2017 at 3:28 AM, Jack Kamm <jackkamm@gmail.com> wrote:
> Yes, I'm starting to see now how difficult it is to properly support
> ":session :results value". I would vote to remove it from ob-python...
>
> I think the patch still improves ":session :results output" so I will
> simplify it and restrict to that case, leaving ":session :results value"
> unchanged for now.
>
> Sorry for sending this twice Kyle, forgot to reply all.
>
> On 21 Nov 2017 4:04 am, "Kyle Meyer" <kyle@kyleam.com> wrote:
>>
>> Jack Kamm <jackkamm@gmail.com> writes:
>>
>> > In response to this:
>> >
>> >> I can't think of a good solution, though.  Stepping back a bit, I think
>> >> it's unfortunate that python blocks handle ":results value" differently
>> >> depending on whether the block is hooked up to a session or not.  For
>> >> non-sessions, you have to use return.  Using the same approach
>> >> (org-babel-python-wrapper-method) for ":session :results value", we
>> >> could then get the return value reliably, but the problem with this
>> >> approach is that any variables defined in a ":results value" block
>> >> wouldn't be defined in the session after executing the block because
>> >> the
>> >> code is wrapped in a function.
>> >
>> > How about if we used the "globals()" and "locals()" functions in Python?
>> >
>> > Something like this at the end of the wrapper block, before return:
>> >
>> > for k, v in locals().items():
>> >     globals()[k] = v
>>
>> Hmm, placing that code "before return" is a problem.  Like with
>> non-session ":results value" blocks, the user would be responsible for
>> inserting the return (or even multiple return's), so we can't know where
>> to insert the above code without parsing the block :/
>>
>> > Another bug with the current approach is that it breaks if common idioms
>> > like "for _ in range(10)" are used. ("_" is used to inspect the last
>> > output
>> > of the shell, an obscure feature I hadn't known about until now).
>>
>> Right.  Also, IIRC the built-in interactive python and ipython treat
>> multiline blocks differently.  With
>>
>>     if True:
>>         "ipython ignores my existence"
>>
>> the built-in shell binds "_" to the string's value, but ipython doesn't.
>>
>> --
>> Kyle