Both Python 2 and 3 in Emacs

Question

I have been using Emacs to write Python 2 code. Now I have both Python 2.6 and 3.0 installed on my system, and I need to write Python 3 code as well.

Here is how the

Answer 1

regarding jrockway's comment:

(defun is-python3-p () "Check whether we're running python 2 or 3."
  (setq mystr (first (split-string (buffer-string) "\n" t)))
  (with-temp-buffer
    (insert mystr)
    (goto-char 0)
    (search-forward "python3" nil t)))
(defun run-python () "Call the python interpreter."
  (interactive)
  (if (is-python3-p)
      (setq py-python-command "/usr/bin/python3")
    (setq py-python-command "/usr/bin/python"))
  (py-execute-buffer))

This will call python3 if "python3" is in the top line of your buffer (shebang, usually). For some reason the (setq py-pyton-command ...) doesn't let you change versions once you've run py-execute-buffer once. That shouldn't be an issue unless you change your file on the same buffer back and forth.