emacs, python-mode.el define-key map assignment

Question

When I load the python-mode.el file in emacs, I don't get any of the key bindings specified. I started playing with the file and noticed that when I change:

(define-key map [(control c)(\#)] 'py-comment-region)

to:

(define-key global-map [(control c)(\#)] 'py-comment-region)

it works fine.

I went to look where the define the map variable and saw:

(defvar py-shell-map nil
  "Keymap used in *Python* shell buffers.")

;; used by py-completion-at-point, the way of python.el
(defvar python-shell-map
  (let ((map (copy-keymap comint-mode-map)))
    (define-key map [tab]   'py-shell-complete)
    (define-key map "\C-c-" 'py-up-exception)
    (define-key map "\C-c=" 'py-down-exception)
    map)
  "Keymap used in *Python* shell buffers.")

Is the 'map' variable defined alright? Should I do some changes to my init file? I assume that this file works to everybody else, so why I need to change the 'map' variable to 'global-map' for it to work in my computer?

I'm running in a virtual machine if that's of any help.

Answer 1

The code you quoted in which you believe map is defined is not actually the relevant portion of the code. It is a different keymap used for a python shell, and it's not the one used when you edit a python file in Emacs.

The line you're editing appears in python-mode inside the following code block:

(defvar python-mode-map)
(setq python-mode-map
      (let ((map (make-sparse-keymap)))
        ;; electric keys
        (define-key map [(:)] 'py-electric-colon)
        (define-key map [(\#)] 'py-electric-comment)
        ...

As you can see the variable map is first initialized as a "sparse keymap", then certain key-bindings get defined in that map, and finally the map is set as the value of python-mode-map. The latter is the keymap used in a buffer that is in python-mode.

So the keybindings should work - but of course only in a buffer that is in python-mode. To activate python-mode in a buffer, type M-x python-mode. This works only after the file python-mode.el has been loaded.

You can check if your current buffer is in python-mode in two ways:

• your mode line should display the letters "Py"
• type M-: ENTER major-mode ENTER -> this should print "python-mode" to the minibuffer

Answer 2

Each major mode and some minor modes have their own keymap, which is overlaid on the global keymap (which is global-map). When you press a key, Emacs tries to find a binding for that key in the overlaid keymaps, falling back to "more global" ones until it gets to the global-map. This is why global-map works and map doesn't.

In lisp, let is used to bind local variables. The map variable doesn't exist outside of the let (or maybe it does, but it's probably not the one you want). Read the documentation for let and defvar (e.g. C-h f defvar).

You need to figure out which keymap is being used in the major mode, and use define-key on that. In this case, (define-key python-mode-map (kbd "C-c #") 'py-comment-region) will probably work. N.B. I do not use python-mode.el, but looking at the source it seems like it uses python-mode-map as the keymap variable. The other keymaps are for auxiliary buffers.