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

[Jack Kamm]

Hi,

Below is a very small patch to Python session blocks, to make them
return a blank result (empty string) instead of None when there is no
return value.

Normally I would push this myself, but since we are so close to 9.4, I
thought it prudent to mail a patch and let the maintainers handle it. It
would be nice to include in 9.4, but not a big deal if I've missed the
window.

Now for an explanation of the patch: 9.4 changes Python session blocks
to fix several bugs and improve robustness overall [0]. However, there
is a cost to these fixes, which is that the session blocks can only
return a result when it is a top-level expression on the last line of
the block. If the last line is not a top-level expression, the block
would previously print "None". However, after some testing, I think this
is a little counter-intuitive, and it would be better if it returned a
blank (empty) result instead of "None".

[0] https://lists.gnu.org/archive/html/emacs-orgmode/2020-01/msg00190.html

Best,
Jack


>From 0b44c3f1c7454e7948cd34eb02995924046b6976 Mon Sep 17 00:00:00 2001
From: Jack Kamm <address@hidden>
Date: Mon, 17 Feb 2020 08:11:49 -0800
Subject: [PATCH] ob-python: Session returns empty string if no return value

* lisp/ob-python.el (org-babel-python--eval-ast): Change sessions to
return an empty string, instead of None, if there is no return value.
---
 etc/ORG-NEWS      | 12 +++++++-----
 lisp/ob-python.el |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/etc/ORG-NEWS b/etc/ORG-NEWS
index b6ee443e4..1b5429870 100644
--- a/etc/ORG-NEWS
+++ b/etc/ORG-NEWS
@@ -12,13 +12,15 @@ Please send Org bug reports to mailto:address@hidden.
 
 * Version 9.4 (not yet released)
 ** Incompatible changes
-*** Python session return values must be top-level expression statements
+*** Change how Python sessions handle return values
 
 Python blocks with ~:session :results value~ header arguments now only
-return a value if the last line is a top-level expression statement,
-otherwise the result is None. Also, None will now show up under
-"#+RESULTS:", as it already did with ~:results value~ for non-session
-blocks.
+return a value if the last line is a top-level expression
+statement. If the last line is not a top-level expression statement,
+the block will return a blank (empty) result.
+
+Also, a return value of ~None~ will now show up under "#+RESULTS:", as
+it already did for non-session blocks.
 
 *** In HTML export, change on how outline-container-* is set
 
diff --git a/lisp/ob-python.el b/lisp/ob-python.el
index de718b04b..b9e36dba0 100644
--- a/lisp/ob-python.el
+++ b/lisp/ob-python.el
@@ -260,7 +260,7 @@ (defconst org-babel-python--eval-ast "\
             __org_babel_python_final.value), '<string>', 'eval'))
     else:
         exec(compile(__org_babel_python_ast, '<string>', 'exec'))
-        __org_babel_python_final = None
+        __org_babel_python_final = ''
 except Exception:
     from traceback import format_exc
     __org_babel_python_final = format_exc()
-- 
2.25.0