delete-trailing-whitespace on save

[address@hidden]

I would like to automatically call delete-trailing-whitespace in
python-mode when I execute save-buffer.  I assume there is some way
using advice, but my emacs lisp skills are very rusty!  At first I
thought I could use an after-save-hook


(add-hook 'python-mode-hook
          (lambda ()
            (setq show-trailing-whitespace t)
            (make-local-hook 'after-save-hook)
            (add-hook 'after-save-hook 'delete-trailing-whitespace)))

but that deletes the whitespace *after* saving.  I need something like
a before-save-hook.  I googled before-save-hook and it appears there
was once such a beast, but it does not appear to be a variable in my
version of emacs (GNU Emacs 21.3.1)

Thanks,
JDH