Python – ny way to format a complete python buffer in emacs with a key press


I am looking for any way to have Emacs format a Python buffer by hitting a few keys. By format, I mean:

  1. Replace tabs with 4 spaces
  2. Wrap all long lines correctly at 79 chars. This includes wrapping & concatenating long strings, wrapping long comments, wrapping lists, function headers, etc.
  3. Unrelated, but when I hit enter, it'd be nice if the cursor was tabbed in automatically.

In general I'd like to just format everything according to PEP 8.

I've looked for a pretty printer / code beautifier / code formatter for Python to run the buffer through, but can't find an open source one.

My .emacs is here.

For those who are going to answer "You don't need a formatter for Python, it's beautiful by the nature of the language," I say to you that this is not correct. In real software systems, comments should be auto-wrapped for you, strings are going to be longer than 79 characters, tab levels run 3+ deep. Please just help me solve my issue directly without some philosophical discussion about the merits of formatting Python source.

Best Solution

To change tabs into spaces and fill comments at the same time, you can use this command:

(defun my-format-python-text ()
  "untabify and wrap python comments"
  (untabify (point-min) (point-max))
  (goto-char (point-min))
  (while (re-search-forward comment-start nil t)
    (call-interactively 'fill-paragraph)
    (forward-line 1)))

Which you can bind to the key of your choice, presumably like so:

(eval-after-load "python"
     (define-key python-mode-map (kbd "RET") 'newline-and-indent)
     (define-key python-mode-map (kbd "<f4>") 'my-format-python-text)))

Note the setting of the RET key to automatically indent.

If you wanted to all tabs with spaces with just built-in commands, this is a possible sequence:

C-x h           ;; mark-whole-buffer
M-x untabify    ;; tabs->spaces

To get the fill column and tab width to be what you want, add to your .emacs:

(setq fill-column 79)
(setq-default tab-width 4)

Arguably, the tab-width should be set to 8, depending on how other folks have indented their code in your environment (8 being a default that some other editors have). If that's the case, you could just set it to 4 in the 'python-mode-hook. It kind of depends on your environment.