Is there a way to autoindent single quoted lines automatically for python code?

Question

The python.el mode has several ways to fill docstrings, but I would like to fill single quoted lines. Is there a function that would change this:

variable = {
    'key': 
    ' this is a very long long long long long long long long long long long long long long long string'}

into this:

variable = {
    'key': 
    ' this is a very long long long long long long long long long long '
    'long long long long long string'}

I do not want to use triple quoted strings, because they add newlines I don't want.

Answer 1

Here's not a complete solution, but will work for many cases:

(defun wvxvw/python-wrap-string ()
  (interactive)
  (save-excursion
    (goto-char (1- (previous-property-change (point))))
    (let ((chop (- fill-column 2 (current-column)))
          (start (point))
          (qmark (buffer-substring-no-properties (point) (1+ (point))))
          (end (1+ (next-property-change (1+ (point)))))
          (padding (make-string (current-column) ?\ )))
      (when (< chop 1) (setf chop 1))
      (let* ((str (buffer-substring-no-properties (1+ start) (1- end)))
             (chunks
              (cl-loop for i from 0 upto (length str) by chop
                       collect (substring str i (min (+ i chop) (length str))))))
        (delete-region start end)
        (insert qmark (car chunks) qmark "\n")
        (cl-loop for s on (cdr chunks) do
                 (insert padding qmark (car s) qmark (if (cdr s) "\n" "")))))))

(define-key python-mode-map (kbd "C-c C-q") 'wvxvw/python-wrap-string)

Some problems with this solution include:

1. This uses SPC unconditionally, while Python mode may have been configured to use TAB for indentation.
2. Genuine fill-paragraph function in Emacs is more intelligent wrt where it splits the string (eg. it will try to split it on word boundaries).
3. The rules for inserting backslash in Python are very difficult to follow. It is possible that in some cases the backslashes will be required, but I'll add this once someone gives me an example where they are required.

Answer 2

What I did eventually was write a python script:

#!/usr/bin/env python3
"""
script for autoindenting single quoted strings.
usuage: provide as stdin e.g.:

        " this is ' a very long line that is too long for the pep8 rules and it also contains a single quote, and the initial indent is included in stdin"

this will output:

        " this is ' a very long line that is too long for the pep8 rules and "
        'it also contains a single quote, and the initial indent is included '
        'in stdin'

with the correct linelength and indentation.
"""
import sys
import re
import pprint


def main():
    """ get input from stdin and print the reformatted string """
    original = sys.stdin.read()
    # parse stdin as code so you get a string that python can format
    exec('global executed_string; executed_string = ({})'.format(original))
    # determine initial indent
    indent = re.match('^ +', original)
    if indent:
        indent = indent.group(0)
    else:
        indent = ''
    pp = pprint.PrettyPrinter(width=79-len(indent))
    text = pp.pformat(executed_string)
    # remove () that is used to paste the single quoted strings together
    text = re.sub(r'(^\()|(\)$)', '', text)
    # add the proper indentation for the first line
    text = re.sub(r'^', indent), text)
    # add the proper indentation for the subsequent lines (pformat add a space
    # at the beginning of each line)
    text = re.sub(r'\n ', '\n{}'.format(indent), text)
    # remove trailing \n
    text = re.sub(r'\n$', '', text)
    print(text)

if __name__ == '__main__':
   main()

And assigned this to an emacs function (http://ergoemacs.org/emacs/elisp_perl_wrapper.html). It may look a bit hacky, but it does what I want it to do. You need to be careful to also select your initial indent in emacs, before you run the script, so it can determine the initial indent. Maybe there's a better way to do that, but for now, this works for me.

Note: because I'm using exec, this might actually give a uge mess if you provide anything other than a string. I might look into doing something about that, but for now, use with care.

Answer 3

Black formatter does this. You can use the blacken.el package. It's available in MELPA.