How to export an octave/matlab array for use by python in org-mode?

Question

I am trying to pass the value of an octave/matlab array into a python code block in org-mode. But I can't get the python-end to receive the value.

With the answers in this question, I can generate an org-mode table from a matlab array a as follows:

#+name: test-data
#+begin_src octave :session test_case :exports both
  a =[ 1 2; 3 4; 5 6];
#+end_src

#+RESULTS: test-data
: org_babel_eoe


#+NAME: test-output-value
#+BEGIN_SRC octave :session test_case :results value :colnames yes :hline yes
  ans = a
#+END_SRC

#+RESULTS: test-output-value
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |

However, when I try to pass the results of the matlab-generated table to python, I get no output from python:

#+NAME: octave-to-python
#+BEGIN_SRC python :var a=test-output-value :results value :exports none
  len(a)
#+END_SRC

#+RESULTS: octave-to-python
: None

Can anyone help explain how to make the chaining above work?

(This is with default Emacs 26, Python 3.8 of Ubuntu 20.04 LTS).

Answer 1

ob-python is also peculiar. Try this:

#+name: test-data
#+begin_src octave :session test_case :results value
  a =[ 1 2; 3 4; 5 6];
  ans = a
#+END_SRC

#+RESULTS: test-data
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |

#+NAME: octave-to-python
#+BEGIN_SRC python :var x=test-data :results value
return len(x)
#+END_SRC

#+RESULTS: octave-to-python
3

ob-python's peculiarity is the different way it treats :results value blocks and :results output blocks: when you specify :results value (which you also get by default if you don't specify anything), then ob-python wraps the source block code in a function, calls the function, gets the result and fills in the #+RESULTS block. That's why you need to have a return in the source block: without it, nothing is returned (actually what is returned is the None object, so that's what you get).

The trouble is that if you decide that you want :results output instead, ob-python decides that in this case, it is not going to wrap the code in a function and if you try to execute it, you get a syntax error: SyntaxError: 'return' outside function. So the same code block gives different results, depending on whether you have specified :results value or :results output - that in itself is perhaps not surprising, since there is no reason that that a result value should be the same as what is printed out, but depending on the code, in one case you get what you expect and in the other case you get a syntax error, because ob-python treats those two cases very differently. E.g.:

#+begin_src python :var a=3 :var b=4
sum = a + b
print("The sum is:", sum)
return sum
#+end_src

#+RESULTS:
: 7

#+begin_src python :var a=3 :var b=4 :results output
sum = a + b
print("The sum is:", sum)
return sum
#+end_src

#+RESULTS:

In the first case (:results value), the output of the print disappears, and the value is 7, so that's what the #+RESULS: block shows. In the second case, the same code block but with :results output specified, gives a syntax error: you have to get rid of the return sum line in order for it to run without error, in which case the result is the output of the print: The sum is 7.

There are technical reasons for this inconsistency, but it is an inconsistency and it is surprising when you first encounter it.