Like suggested in this question How to automatically run inferior process when loading major mode for the first time? I want to automatically run run-python when entering python major mode. However,

(add-hook 'python-mode-hook 'run-python)

doesn't work and gives the error

Wrong number of arguments: (1 . 3), 0

Which argument is needed by run-python? How to automatically start an inferior python interpreter when entering python-mode?

Edit: After M-x toggle-debug-on-error the output gives

Debugger entered--Lisp error: (wrong-number-of-arguments (1 . 3) 0)
run-hooks(change-major-mode-after-body-hook prog-mode-hook python-mode-hook)
apply(run-hooks (change-major-mode-after-body-hook prog-mode-hook python-mode-hook))
call-interactively(python-mode record nil)
command-execute(python-mode record)
execute-extended-command(nil "python-mode")
call-interactively(execute-extended-command nil nil)
  • 1
    What you tried would actually work. Technically run-python doesn't require any arguments, at least not in the version of Emacs that I have. You could do C-h f run-python to see what's the function signature you have, though I'm almost sure the error comes from some other place. M-x toggle-debug-on-error would help to find the problem.
    – wvxvw
    Sep 5, 2015 at 16:42
  • I'm using Emacs ver. 24.5.1 and also tried an empty init.el with only (add-hook 'python-mode-hook 'run-python) in it. C-h f run-python gives the signature (run-python CMD &optional DEDICATED SHOW). For debug output see edited question above. I tried an Emacs ver. 23.4.1 on a different machine and indeed there is (add-hook 'python-mode-hook 'run-python) working as expected.
    – max0r
    Sep 5, 2015 at 17:04
  • For some reason on your machine run-python requires CMD argument... well, if that's the case you could change your code to (add-hook 'python-mode-hook (lambda () (run-python (python-shell-calculate-command))).
    – wvxvw
    Sep 5, 2015 at 17:06

1 Answer 1


If you look at documentation of run-python, C-hfrun-python you would notice that requires on parameter cmd. When that command is called interactively emacs calculates that parameter (see Using interactive or C-hfinteractive to understand the mechanism). However when called from elisp, the cmd need to be explicitly passed.

There are couple of ways to solve this -

1) Write a custom function which calls run-python with proper arguments, something like following, and add it to python-mode-hook

(defun my-run-python ()
    (run-python (python-shell-parse-command)))


After understanding your problems with the above approach I came up with the following function, hopefully this is what you are looking for

(defun my-run-python ()
    (switch-to-buffer-other-window (process-buffer (python-shell-get-or-create-process (python-shell-parse-command))))))

2) Add the function python-shell-switch-to-shell which does not expect any arguments, to python-mode-hook

  • Is it possible to see which arguments are passed to run-python when called interactively? Neither your first suggestion nor the second gives the same result as calling run-python or pressing C-c C-p. The first one doesn't create a new window with the python process (I'll have to do it manually) and the second asks me to manually input the parameters every time.
    – max0r
    Sep 6, 2015 at 9:38
  • I accomplished it with my basic knowledge of Emacs Lisp: (defun run-python-once () (remove-hook 'python-mode-hook 'run-python-once) (switch-to-buffer (last-buffer)) (run-python (python-shell-parse-command)) (split-window-sensibly) (switch-to-buffer-other-window (python-shell-get-buffer)) ) added to python-mode-hook. But that doesn't seem pretty straightforward to me.
    – max0r
    Sep 6, 2015 at 11:52
  • The problem with (lambda () (run-python (python-shell-parse-command) nil t)) added to python-mode-hook is, that an opened file doesn't show up. A new window is created and the inferior python process is shown, but the first window dosen't change to the newly opened file buffer.
    – max0r
    Sep 6, 2015 at 12:49
  • @max0r I have edited the answer to mention an alternative approach hopefully it resolves your current issues. Sep 6, 2015 at 13:13

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