py-extract-variable.el

;; This pretty much works.  A couple of remaining quibbles:
;; 
;; - it’s possible for the region to not include a single Python
;;   expression, in which case the code transformation will be
;;   invalid.
;; - it should error out if called interactively when
;;   transient-mark-mode is active and the mark is inactive, but it
;;   doesn’t, because I don't know how to do that.
;; - it would be nice to highlight the other occurrences of the
;;   expression, if any, and provide a key (maybe the same key?) to
;;   replace them too, since most commonly you extract a local
;;   variable when the same expression occurs more than once.  Maybe
;;   using overlays?  How does highlight-regexp from hi-lock work?
;; - Inside a lambda or other binding construct (like a genex), it can
;;   make invalid transformations.
(defun py-extract-variable (start end)
  "Simple extract-variable refactoring for Python.

The idea is that you set the region to the expression to
extract (probably with \\[mark-sexp], whose more usual convenient
keybinding is C-M-SPC) and put the variable name before it.  So
to turn something into a variable \"foo\", you go before it, type
\"foo\", \\[mark-sexp] a few times, and then
\\[py-extract-variable].

"

  (interactive "r")
  (save-excursion
    (let ((expression (buffer-substring start end)))
      (delete-region start end)
      (goto-char start)
      (delete-horizontal-space)         ; in case there was whitespace
      (let ((variable-end (point)))
        (backward-sexp)                 ; find the beginning of the variable
        (let ((variable (buffer-substring (point) variable-end)))
          (python-previous-statement 0)
          ;; I can’t get (indent-region) and (indent-for-tab-command)
          ;; to work properly, so I’ll just brute-force inserting the
          ;; same indent.
          (let ((end-of-indent (point)))
            (move-beginning-of-line 1)
            (let ((indent (buffer-substring (point) end-of-indent)))
              (open-line 1)
              (insert indent)
              (insert variable)
              (insert " = ")
              (insert expression))))))))

(require 'python)
(define-key python-mode-map [f6] 'py-extract-variable)