ob-python: Fix several issues with :session :results value

* lisp/ob-python.el (org-babel-python-evaluate-session): Fix a few
related issues with :session :results value blocks, including broken
if-else statements, indented blocks with blank lines, and returning
the wrong value when underscore has been used.
(org-babel-python--eval-ast): New constant variable, a string
consisting of Python code to execute a source block using ast.

Previously, python blocks with parameters ":session :results value"
were entered line-by-line into the Python session, which could cause
issues around indentation and new lines.  Now, such python blocks are
written to temp files, then the built-in ast python module is used to
parse and execute them, and to extract the last line separately to
return as a result.  Introduces a change in behavior, requiring that
the last line must be a top-level expression statement if its result
is to be saved (otherwise, the result is None).

etc/ORG-NEWS

@@ -11,6 +11,15 @@ See the end of the file for license conditions.
 Please send Org bug reports to mailto:emacs-orgmode@gnu.org.
 
 * Version 9.4 (not yet released)
+** Incompatible changes
+*** Python session return values must be top-level expression statements
+
+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.
+
 ** New features
 
 *** Numeric priorities are now allowed (up to 65)

lisp/ob-python.el

@@ -247,6 +247,25 @@ open('%s', 'w').write( pprint.pformat(main()) )")
    ")); "
    "__org_babel_python_fh.close()"))
 
+(defconst org-babel-python--eval-ast "\
+import ast
+try:
+    with open('%s') as f:
+        __org_babel_python_ast = ast.parse(f.read())
+    __org_babel_python_final = __org_babel_python_ast.body[-1]
+    if isinstance(__org_babel_python_final, ast.Expr):
+        __org_babel_python_ast.body = __org_babel_python_ast.body[:-1]
+        exec(compile(__org_babel_python_ast, '<string>', 'exec'))
+        __org_babel_python_final = eval(compile(ast.Expression(
+            __org_babel_python_final.value), '<string>', 'eval'))
+    else:
+        exec(compile(__org_babel_python_ast, '<string>', 'exec'))
+        __org_babel_python_final = None
+except Exception:
+    from traceback import format_exc
+    __org_babel_python_final = format_exc()
+    raise")
+
 (defun org-babel-python-evaluate
   (session body &optional result-type result-params preamble)
   "Evaluate BODY as Python code."
@@ -294,32 +313,9 @@ If RESULT-TYPE equals `output' then return standard output as a
 string.  If RESULT-TYPE equals `value' then return the value of the
 last statement in BODY, as elisp."
   (let* ((send-wait (lambda () (comint-send-input nil t) (sleep-for 0 5)))
-	 (dump-last-value
-	  (lambda
-	    (tmp-file pp)
-	    (mapc
-	     (lambda (statement) (insert statement) (funcall send-wait))
-	     (if pp
-		 (list
-		  "import pprint"
-		  (format "open('%s', 'w').write(pprint.pformat(_))"
-			  (org-babel-process-file-name tmp-file 'noquote)))
-	       (list (format "open('%s', 'w').write(str(_))"
-			     (org-babel-process-file-name tmp-file
-                                                          'noquote)))))))
 	 (last-indent 0)
 	 (input-body (lambda (body)
 		       (dolist (line (split-string body "[\r\n]"))
-			 ;; Insert a blank line to end an indent
-			 ;; block.
-			 (let ((curr-indent (string-match "\\S-" line)))
-			   (if curr-indent
-			       (progn
-				 (when (< curr-indent last-indent)
-				   (insert "")
-				   (funcall send-wait))
-				 (setq last-indent curr-indent))
-			     (setq last-indent 0)))
 			 (insert line)
 			 (funcall send-wait))
 		       (funcall send-wait)))
@@ -344,17 +340,35 @@ last statement in BODY, as elisp."
 		   (funcall send-wait))
 		 2) "\n")))
             (`value
-             (let ((tmp-file (org-babel-temp-file "python-")))
+             (let ((tmp-results-file (org-babel-temp-file "python-"))
+		   (body (let ((tmp-src-file (org-babel-temp-file
+					      "python-")))
+			   (with-temp-file tmp-src-file (insert body))
+			   (format org-babel-python--eval-ast
+				   tmp-src-file))))
                (org-babel-comint-with-output
                    (session org-babel-python-eoe-indicator nil body)
                  (let ((comint-process-echoes nil))
                    (funcall input-body body)
-                   (funcall dump-last-value tmp-file
-                            (member "pp" result-params))
+		   (dolist
+		       (statement
+			(if (member "pp" result-params)
+			    (list
+			     "import pprint"
+			     (format "open('%s', 'w').write(pprint.pformat(\
+__org_babel_python_final))"
+				     (org-babel-process-file-name
+				      tmp-results-file 'noquote)))
+			  (list (format "open('%s', 'w').write(str(\
+__org_babel_python_final))"
+					(org-babel-process-file-name
+					 tmp-results-file 'noquote)))))
+		     (insert statement)
+		     (funcall send-wait))
                    (funcall send-wait) (funcall send-wait)
                    (insert org-babel-python-eoe-indicator)
                    (funcall send-wait)))
-               (org-babel-eval-read-file tmp-file))))))
+               (org-babel-eval-read-file tmp-results-file))))))
     (unless (string= (substring org-babel-python-eoe-indicator 1 -1) results)
       (org-babel-result-cond result-params
 	results

testing/lisp/test-ob-python.el

@@ -138,6 +138,41 @@ if True:
 	      (org-babel-execute-maybe)
 	      (org-babel-execute-src-block)))))
 
+(ert-deftest test-ob-python/if-else-block ()
+  (should
+   (equal "success" (org-test-with-temp-text "#+begin_src python :session :results value
+value = 'failure'
+if False:
+    pass
+else:
+    value = 'success'
+value
+#+end_src"
+	      (org-babel-execute-src-block)))))
+
+(ert-deftest test-ob-python/indent-block-with-blank-lines ()
+  (should
+   (equal 20
+	  (org-test-with-temp-text "#+begin_src python :session :results value
+  foo = 0
+  for i in range(10):
+      foo += 1
+
+      foo += 1
+
+  foo
+#+end_src"
+	    (org-babel-execute-src-block)))))
+
+(ert-deftest test-ob-python/assign-underscore ()
+  (should
+   (equal "success"
+	  (org-test-with-temp-text "#+begin_src python :session :results value
+_ = 'failure'
+'success'
+#+end_src"
+	    (org-babel-execute-src-block)))))
+
 (provide 'test-ob-python)
 
 ;;; test-ob-python.el ends here