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Emacs: Auto-format strings to a max column width in `python-mode`

I use blacken-mode (uses black) to auto-format Python code on save -- it works great but not much helpful while dealing with strings. For example, if I want to set a max limit on columns (characters) per line when writing a string, black is of no help. So I needed to come up with something in emacs that does the auto-format of strings -- set a max limit on-the-fly on number of columns per line for writing strings.

I came up with the following elisp function to do the above:

(defun local-auto-format-max-column-width-string-python-mode (&optional max-column-width)
  "Auto-format lines containing strings in `python-mode` to `MAX-COLUMN-WIDTH' characters.

If not provided, the `MAX-COLUMN-WIDTH' defaults to 88 characters/columns.

As column numbers in emacs starts from 0, it does the check for the current line at
(+ `MAX-COLUMN-WIDTH' 1) to avoid side effects when jumping through the code rather
 than actually typing.

When the limit is reached:
- if the string is inside a function (e.g. `print'), it moves the start of the string
 (with any existing string-prefixes and quote) to the next line, then moves the `point'
 to the end
- if not inside a function, closes the current line at `MAX-COLUMN-WIDTH' with the
 initially used quote (and any existing string-prefixes), jumps to the correctly
 prefixed-quoted-indented next line, and sets the `point'"

  (when (equal major-mode 'python-mode)
    (setq max-column-width (or max-column-width 88))
    (when (equal (current-column) (+ max-column-width 1))
           (end (point))

        (setq start (point))
        (goto-char end)
        (setq selection (buffer-substring-no-properties start end))

        ;; Only do the auto-formatting when we're on an unclosed single/double quote (string)
        (when (string-match "^.*\\(['\"]\\)[^'\"]+\\'" selection)
          (setq current-string-quote (match-string 1 selection))

          ;; String is inside a function call e.g. `print("...`
          (if (string-match "^[[:blank:]]*[^[:blank:]]+([[:blank:]]*\\([A-Za-z]*['\"][^'\"]+\\'\\)" selection)
                (setq quoted-string-obj (match-string 1 selection))
                 (- (point) (length quoted-string-obj)))

            ;; Line containing only the string (possibly with preceding whitespaces)
            (goto-char (- end (if current-string-quote 2 1)))
            (when current-string-quote
              (insert current-string-quote))
            ;; Indentation
            (when (string-match "^\\([[:blank:]]*\\(?:[A-Za-z]*['\"]\\)?\\)[^'\"]+\\'" selection)
              (insert (match-string 1 selection)))

This needs to be added as a post-command-hook so that every keystroke can be monitored to apply it on the right column:

(add-hook 'post-command-hook #'local-auto-format-max-column-width-string-python-mode)

Also, the above add-hook can only be run as a python-mode-hook to ensure that it's not invoked unnecessarily beforehand e.g.:

(add-hook 'python-mode-hook (lambda () (add-hook 'post-command-hook #'local-auto-format-max-column-width-string-python-mode)))

(I've used lambda as an example above but a named function rather than a lambda would be more appropriate as the hook function -- if we need to remove the hook at some point, the remove-hook function needs the exact function used in add-hook.)

Note: As a post-command-hook is run after every command, we need to make sure the hook is not clogging up the command loop of emacs. From my tests, I don't see any slowness or any other issues in emacs that could be relevant so works for me.


  1. blacken-mode
  2. black
  3. post-command-hook
  4. add-hook
  5. remove-hook
  6. Python string prefixes


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