From 17cf2c710aaa3dda657f01ce8e77e2043725372e Mon Sep 17 00:00:00 2001
From: toctan <tianjin.sc@gmail.com>
Date: Wed, 20 Aug 2014 16:14:05 +0800
Subject: [PATCH] Insert magic encoding comment upon save

---
 modules/prelude-python.el | 42 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/modules/prelude-python.el b/modules/prelude-python.el
index 604ef87..1f7a44e 100644
--- a/modules/prelude-python.el
+++ b/modules/prelude-python.el
@@ -35,6 +35,45 @@
 
 (require 'prelude-programming)
 
+;; Copy pasted from ruby-mode.el
+(defun prelude-python--encoding-comment-required-p ()
+  (re-search-forward "[^\0-\177]" nil t))
+
+(defun prelude-python--detect-encoding ()
+  (let ((coding-system
+         (or save-buffer-coding-system
+             buffer-file-coding-system)))
+    (if coding-system
+        (symbol-name
+              (or (coding-system-get coding-system 'mime-charset)
+                  (coding-system-change-eol-conversion coding-system nil)))
+      "ascii-8bit")))
+
+(defun prelude-python--insert-coding-comment (encoding)
+  (let ((newlines (if (looking-at "^\\s *$") "\n" "\n\n")))
+    (insert (format "# coding: %s" encoding) newlines)))
+
+(defun prelude-python-mode-set-encoding ()
+  "Insert a magic comment header with the proper encoding if necessary."
+  (save-excursion
+    (widen)
+    (goto-char (point-min))
+    (when (prelude-python--encoding-comment-required-p)
+      (goto-char (point-min))
+      (let ((coding-system (prelude-python--detect-encoding)))
+        (when coding-system
+          (if (looking-at "^#!") (beginning-of-line 2))
+          (cond ((looking-at "\\s *#\\s *.*\\(en\\)?coding\\s *:\\s *\\([-a-z0-9_]*\\)")
+                 ;; update existing encoding comment if necessary
+                 (unless (string= (match-string 2) coding-system)
+                   (goto-char (match-beginning 2))
+                   (delete-region (point) (match-end 2))
+                   (insert coding-system)))
+                ((looking-at "\\s *#.*coding\\s *[:=]"))
+                (t (prelude-python--insert-coding-comment coding-system)))
+          (when (buffer-modified-p)
+            (basic-save-buffer-1)))))))
+
 (defun prelude-python-mode-defaults ()
   "Defaults for Python programming."
   (subword-mode +1)
@@ -42,7 +81,8 @@
 	      '((?: . (lambda ()
                         (if (python-info-statement-starts-block-p)
                             'after)))))
-  (electric-layout-mode +1))
+  (electric-layout-mode +1)
+  (add-hook 'after-save-hook 'prelude-python-mode-set-encoding nil 'local))
 
 (setq prelude-python-mode-hook 'prelude-python-mode-defaults)