Python-mode Multi-line Conditional Indentation

Question

Given the following somewhat silly Python code with a multi-line condition:

if ("VALUE" in my_uncessarily_long_dictionary and
    some_other_long_condition_case):
    do_something()
else:
    pass

This is written as Emacs typically indents each line, which I'm normally fine with. However, quite a few linters prefer the condition to be indented visually different from the next (non-conditional) line, e.g., something like this:

if ("VALUE" in my_uncessarily_long_dictionary and
        some_other_long_condition_case):
    do_something():
else:
    pass

Is there some way to configure python-mode to indent like this?

Answer 1

The following lines within python-indent--calculate-indentation determine the indent after a paren not following a newline:

(defun python-indent--calculate-indentation ()
  ...
  (pcase (python-indent-context)
    ...
    (`(,(or :inside-paren
            :after-backslash-block-continuation
            :after-backslash-dotted-continuation) . ,start)
     ;; Use the column given by the context.
     (goto-char start)
     (current-column))
    ...
    )
  )

The result of the form (current-column) is the calculated indentation. It is hard-coded and changing the indentation after a paren requires changing the code.
You could do that in the following way:

(defcustom phyton-indent-add-after-paren python-indent-offset
  "Additional indent after paren."
  :type 'number
  :group 'python)

(defun python-indent--calculate-indentation ()
  ...
  (pcase (python-indent-context)
    ...
    (`(:inside-paren . ,start)
     (goto-char start)
     (+ (current-column) phyton-indent-add-after-paren))
    (`(,(or :after-backslash-block-continuation
            :after-backslash-dotted-continuation) . ,start)
     ;; Use the column given by the context.
     (goto-char start)
     (current-column))
    ...
    )
  )

---

Alternatively, you could use the following advice in your init file:

(defcustom phyton-indent-add-after-paren python-indent-offset
  "Additional indent after paren without following newline."
  :type 'number
  :group 'python)

(defun python-indent-calc-indent-ad (fun)
  "Change indent after opening paren with `phyton-indent-add-after-paren'.
This is an around advice for `python-indent--calculate-indentation'."
  (save-excursion
    (let* ((info (python-indent-context))
       (ctx (car info))
       (start (cdr info)))
      (if (eq ctx :inside-paren)
      (progn
        (goto-char start)
        (+ (current-column) python-indent-offset))
    (funcall fun)))))

(advice-add 'python-indent--calculate-indentation :around #'python-indent-calc-indent-ad)

This is less efficient than the modification of python-indent--calculate-indentation since the heavy duty function python-indent-context is called twice. But if you do not notice the slow down, it should be okay.

---

Note, this is valid for Emacs 28.1. It may be that other Emacs versions require other changes.

You could issue a feature request at debbugs. See an SE-answer about feature requests.

Answer 2

This indentation behaviour can now be configured as of commit 711e8bc7178d5dd78f4db5c34b2b23f605521fc4, (or Emacs-29 and above I think).

To use it, simply set the python-indent-block-paren-deeper option to t if you want to enable this indentation behaviour, e.g.:

(setq python-indent-block-paren-deeper t)