#20440 reproducible code

test-revert-buffer.el

(require 'cl-lib)

(defun my/test-revert-buffer ()
  (let ((file (make-temp-file "test-revert-buffer" nil ".py"))
        (file-content1 "# -*- coding: utf-8 -*-


'''
Foo.
'''
")
        (file-content2 "# -*- coding: utf-8 -*-

class Foo(object):
    pass
"))
    (write-region file-content1 nil file nil 'silent)
    (find-file file)
    ;; The content of the current buffer is equal to file-content1.

    (write-region file-content2 nil file nil 'silent)
    (revert-buffer nil t)
    ;; Now the content of the current buffer should be equal to file-content2.
    (cl-assert (string= (buffer-string) file-content2))))

(defun my/add-to-after-change-functions ()
  (add-hook 'after-change-functions (lambda (x y z) (buffer-string)) nil t))

(add-hook 'python-mode-hook #'my/add-to-after-change-functions)

(my/test-revert-buffer)