Chris R Chris R - 4 months ago 9
Python Question

How can I access directory-local variables in my major mode hooks?

I have defined a .dir-locals.el file with the following content:

((python-mode . ((cr/virtualenv-name . "saas"))))


In my .emacs I have the following function to retrieve this value and provide a virtualenv path:

(defun cr/virtualenv ()
(cond (cr/virtualenv-name (format "%s/%s" virtualenv-base cr/virtualenv-name))
((getenv "EMACS_VIRTUAL_ENV") (getenv "EMACS_VIRTUAL_ENV"))
(t "~/.emacs.d/python")))


Finally, in my python-mode-hook list, I have this hook function:

(add-hook 'python-mode-hook 'cr/python-mode-shell-setup)

(defun cr/python-mode-shell-setup ()
(message "virtualenv-name is %s" cr/virtualenv-name)
(let ((python-base (cr/virtualenv)))
(cond ((and (fboundp 'ipython-shell-hook) (file-executable-p (concat python-base "/bin/ipython")))
(setq python-python-command (concat python-base "/bin/ipython"))
(setq py-python-command (concat python-base "/bin/ipython"))
(setq py-python-command-args '( "-colors" "NoColor")))
(t
(setq python-python-command (concat python-base "/bin/python"))
(setq py-python-command (concat python-base "/bin/python"))
(setq py-python-command-args nil)))))


When I open a new python file, the message logged by
cr/python-mode-shell-setup
indicates that
cr/virtualenv-name
is
nil
. However, when I C-h v the name, I get "saas" instead.

Obviously there's a load order issue here; is there a way to have my mode hook statements respond to directory-local variables?

Answer

This happens because normal-mode calls (set-auto-mode) and (hack-local-variables) in that order.

However hack-local-variables-hook is run after the local variables have been processed, which enables some solutions:

  1. The first is to make Emacs run a new "local variables hook" for each major mode:

    (add-hook 'hack-local-variables-hook 'run-local-vars-mode-hook)
    (defun run-local-vars-mode-hook ()
      "Run a hook for the major-mode after the local variables have been processed."
      (run-hooks (intern (concat (symbol-name major-mode) "-local-vars-hook"))))
    
    (add-hook 'python-mode-local-vars-hook 'cr/python-mode-shell-setup)
    

    (Your original function can be used unmodified, with that approach.)

  2. A second option is to utilise the optional LOCAL argument to add-hook that makes the specified function buffer-local. With this approach you could write your hook as follows:

    (add-hook 'python-mode-hook 'cr/python-mode-shell-setup)
    
    (defun cr/python-mode-shell-setup ()
      (add-hook 'hack-local-variables-hook
                (lambda () (message "virtualenv-name is %s" cr/virtualenv-name)
                  (let ((python-base (cr/virtualenv)))
                    (cond ((and (fboundp 'ipython-shell-hook) (file-executable-p (concat python-base "/bin/ipython")))
                           (setq python-python-command (concat python-base "/bin/ipython"))
                           (setq py-python-command (concat python-base "/bin/ipython"))
                           (setq py-python-command-args '( "-colors" "NoColor")))
                          (t
                           (setq python-python-command (concat python-base "/bin/python"))
                           (setq py-python-command (concat python-base "/bin/python"))
                           (setq py-python-command-args nil)))))
                nil t)) ; buffer-local hack-local-variables-hook
    

    i.e. python-mode-hook runs first and registers the anonymous function with hack-local-variables-hook for the current buffer only; and that function is then called after the local variables have been processed.

  3. Lindydancer's comment prompts a third approach. It's not nearly as clean as the other two, but proved interesting regardless. I didn't like the idea of causing (hack-local-variables) to be called twice, but I see that if you set the local-enable-local-variables buffer-locally, it prevents (hack-local-variables) from doing anything, so you could do this:

    (defun cr/python-mode-shell-setup ()
      (report-errors "File local-variables error: %s"
        (hack-local-variables)))
      (set (make-local-variable 'local-enable-local-variables) nil)
      (let ((python-base (cr/virtualenv)))
        ...))
    

    Obviously that modifies the normal sequence of execution a little, so side effects may be possible. I was worried that if the same major mode is set by a local variable comment in the file, this might cause infinite recursion, but that doesn't actually appear to be a problem.

    Local variable header comments (e.g. -*- mode: foo -*-) are handled by (set-auto-mode), so those are fine; but a mode: foo Local Variables: comment seems like it would be an issue as it is handled by (hack-local-variables), and so if the mode is set that way I thought it would cause recursion.

    In practice I was able to trigger the problem by using a simple function as a 'mode' which did nothing more than try to run its hooks; however testing with a 'proper' mode did not exhibit the problem, so it's probably safe in reality. I didn't look into this further (as the other two solutions are much cleaner than this), but I would guess the delayed mode hooks mechanism probably explains it?