auto-complete for python-mode using python-shell-completion

gistfile1.el

(defun ac-python-candidates ()
  (let* (py-split-windows-on-execute-p
         py-switch-buffers-on-execute-p
         (shell (py-choose-shell))
         (proc (or (get-buffer-process shell)
                   (get-buffer-process (py-shell nil nil shell 'noswitch nil)))))
    (if (processp proc)
        (with-syntax-table python-dotty-syntax-table
          (let* ((imports (py-find-imports))
                 (beg (save-excursion (skip-chars-backward "a-zA-Z0-9_.") (point)))
                 (end (point))
                 (word (buffer-substring-no-properties beg end))
                 (code (if imports
                           (concat imports python-shell-module-completion-string-code)
                         python-shell-module-completion-string-code)))
            (python-shell-completion--get-completions word proc code))))))

(ac-define-source python
  '((candidates . ac-python-candidates)
    (symbol . "f")
    (cache)))