how to automatically clean indented blank lines?

Question

In python mode when I'm done with an inner indented block I hit return, then backspace to return to the previous indentation and then another return so that there's a blank line before I start the code following that indented block. This leaves a blank line that has only tabs or spaces. I try and find them and remove the tabs and spaces with M-\ but it would be more convenient if there was a save file hook that would find all of them and do this cleanup. Is there something that does that?

Answer 1

Call delete-trailing-whitespace from before-save-hook:

(defun my-python-mode-setup ()
  (add-hook 'before-save-hook #'delete-trailing-whitespace nil 'local)
  ;; Maybe this could be helpful too
  (local-set-key (kbd "RET") #'reindent-then-newline-and-indent))
(add-hook 'python-mode-hook #'my-python-mode-setup)

This setting makes delete-trailing-whitespace run on save but only in in Python Mode buffers. The binding makes RET reindent the current line (which should remove all indentation in an empty line) before making a new one.

(You can set delete-trailing-lines to nil to suppress the deletion of trailing empty lines, if you want.)