Subject: 30.0.50; python-fill-paragraph problems on filling strings
From: kobarity <[email-masked]>
To: [email-masked]
Cc: Stefan Monnier <[email-masked]>,João Távora <[email-masked]>
Date: Sun, 12 Mar 2023 17:39:37 +0900
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi,

There are some problems in filling Python's strings, which are caused
by the same portion of the code.

Here are the steps to reproduce the first problem:

1. emacs -Q
2. C-x C-f somefile1.py
3. Enter the following program:

#+begin_src python
s1 = 'abc'
s2 = 'def'
#+end_src

4. Place point within the first string (abc).
6. M-q

The lines will be merged into a single line.

#+begin_src python
s1 = 'abc' s2 = 'def'
#+end_src

This problem occurs in Emacs 27 and later.

Second problem can be reproduced with the following steps:

1. emacs -Q
2. C-x C-f somefile2.py
3. Enter the following program:

#+begin_src python
s = '''abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz'''
#+end_src

4. Place point at the first or third quote of the starting triple quotes.
5. M-q

The line is not filled.  However, if the point is placed at the second
quote or within the string, the line will be filled as follows:

#+begin_src python
s = '''abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz
abcdefghijklmnopqrstuvwxyz'''
#+end_src

When the point is placed at the ending triple quotes, the line will be
filled if the point is at the first or third quote.

This problem occurs in Emacs 28 and later.

Third problem can be reproduced with the following steps:

1. emacs -Q
2. C-x C-f somefile3.py
3. Enter the following program:

#+begin_src python
s = 'abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz'
#+end_src

4. Place point within the string.
5. M-q

The line will be split into 2 lines.

#+begin_src python
s = 'abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz
abcdefghijklmnopqrstuvwxyz'
#+end_src

This is not a correct Python program anymore because a single quoted
string can not span multiple lines unless the end of line is escaped
using a backslash.

The docstring of `python-fill-paragraph' says:
`fill-paragraph-function' handling multi-line strings and possibly comments.

Although it is possible to make a multi-line single quoted string by
escaping the end of line using backslash, triple quoted strings are
usually used for multi-line strings.  So I think
`python-fill-paragraph' should not do anything with single quoted
strings.

The cause of the first problem is similar to Bug#58780.  In
`python-fill-string', the string is searched for with the following
code:

           (or (python-syntax-context 'string)
               (and (equal (string-to-syntax "|")
                           (syntax-after (point)))
                    (point)))))

and the end of the string is searched for with the following code:

            (or (re-search-forward (rx (syntax string-delimiter)) nil t)
                (goto-char (point-max)))

These codes used to be appropriate, but it is not correct after the
following commit which is included in Emacs 27 and later:

commit 4b39b741f1949ebad1dfccc5032dfce521bedc2a
Author: Stefan Monnier <[email-masked]>
Date:   Tue Apr 9 15:08:21 2019 -0400

    python.el: don't syntax-propertize single/double quoted strings

    * lisp/progmodes/python.el (python-syntax-propertize-function):
    Only mark triple-quoted strings, let the normal syntax-table handle
    the rest.
    (python-syntax-stringify): Adjust accordingly.

From this commit, single quoted strings can not be identified by
string-delimiter and can only be identified by string-quote.

In the fix of Bug#58780, I added `forward-sexp' to search for the end
of single quoted string.  However, considering the third problem, I
would prefer to have `python-fill-paragraph' and `python-fill-string'
search only for triple quoted strings in this case.

The second problem occurs after the following commit included in Emacs
28 and later:

commit 0646c6817139aa905a2f6079fdc82eb4be944de0
Author: João Távora <[email-masked]>
Date:   Sun Sep 19 11:42:20 2021 +0100

    Make syntax-ppss more accurate for Python triple quotes (bug#49518)

    By putting delimiter syntax on the last character of Python
    triple-quoted strings, this makes syntax-ppss be more accurate.

This commit narrows the range of string-delimiter from outermost
quotes to innermost quotes, and results in `python-fill-paragraph' not
recognizing the string when point is at some quotes.

Attached is a patch to fix these problems.  Main changes are:

1. single-quoted-string and triple-quoted-string are added as a TYPE
   argument of `python-syntax-context'.
2. New helper function `python-info-triple-quoted-string-p' is added
   to detect triple quoted strings including quotes.
3. `python-fill-paragraph' and `python-fill-string' use
   `python-info-triple-quoted-string-p' to detect triple quoted
   strings only.

I look forward to your comments.

--
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