org-babel python code block - not showing errors during execution

Question

I have a python code block in an org-file that when executed results in some error. I was expecting to see that error in the #+RESULTS: block, but it doesn't show up. I have to navigate to the python session buffer in order to see whether the evaluation was successful. More over, when a previous code block is unsuccessful, and the next code block depends on it's results, I can continue executing code without any indications that evaluation is failing (except for missing output).

How can I configure org-babel to display such errors/exceptions in the #+RESULTS: block?

How to reproduce

I have this property set at the top of my org file

#+PROPERTY: header-args :session my_py_session

Sample block

#+BEGIN_SRC python
r = some_function_which_results_in_a_trace_back()
#+END_SRC

#+BEGIN_SRC python
x = r*2
#+END_SRC

When I do a C-c on both these blocks, Emacs reports something like Code block evaluation complete (took 0.5 secs) in the mini buffer.

But when I open the python session buffer, I see this error. How can I get this error to show in the #+RESULTS: block?

>>> 
org_babel_python_eoe
Traceback (most recent call last):
  File "<string>", line 17, in __PYTHON_EL_eval
  File "/var/folders/7k/r1g1zhmx16gbtm1hbllhhk_w0000gs/T/py3Fvne8", line 18, in <module>
  File "<string>", line 4, in <module>
  File "<string>", line 2, in <module>
NameError: name 'some_function_which_results_in_a_trace_back' is not defined. Did you mean: 'NameError'?
>>> 
org_babel_python_eoe
Traceback (most recent call last):
  File "<string>", line 17, in __PYTHON_EL_eval
  File "/var/folders/7k/r1g1zhmx16gbtm1hbllhhk_w0000gs/T/pywWfEwY", line 8, in <module>
  File "<string>", line 1, in <module>
NameError: name 'r' is not defined

What I already tried

• Tried redirecting sys.stderr to sys.stdout
• Tried this: Display errors and warnings in an org-mode code block
• Explicitly try-catching works, but is very tedious to do for every block

#+BEGIN_SRC python :results output
try:
    r = some_error()
except Exception as e:
    print(e)
#+END_SRC

#+RESULTS:
: name 'some_error' is not defined

Answer 1

Do you want the whole stacktrace? Or are you fine with just the error message?

Override this function to your heart's content:

(advice-add #'org-babel-python-send-string 
            :override
            #'my/org-babel-python-send-string)

(defun my/org-babel-python-send-string (session body)
  "Pass BODY to the Python process in SESSION.
Return output."
  (with-current-buffer session
    (let* ((string-buffer "")
       (comint-output-filter-functions
        (cons (lambda (text) (setq string-buffer
                       (concat string-buffer text)))
          comint-output-filter-functions))
       (body (format "\
try:
%s
except Exception as e:
    # These are the relevant lines to change. 
    print(e)
finally:
    print('%s')"
             (org-babel-python--shift-right body 4)
             org-babel-python-eoe-indicator)))
      (let ((python-shell-buffer-name
         (org-babel-python-without-earmuffs session)))
    (python-shell-send-string body))
      ;; same as `python-shell-comint-end-of-output-p' in emacs-25.1+
      (while (not (and (python-shell-comint-end-of-output-p string-buffer)
                       (string-match
                org-babel-python-eoe-indicator
                string-buffer)))
    (accept-process-output (get-buffer-process (current-buffer))))
      (org-babel-chomp (substring string-buffer 0 (match-beginning 0))))))

You'll also need to change:

#+PROPERTY: header-args :session my_py_session

to

#+PROPERTY: header-args :session my_py_session :results output

Insert the appropriate python code if you want the whole stacktrace.