New feature: when saving a file, keep the eol convention of the

original.  New files are written using the eol convention of the
platform, given by os.linesep.  All files are read and written in
binary mode.

Lib/idlelib/IOBinding.py

@@ -178,6 +178,10 @@ class IOBinding:
             self.text.focus_set()
         return "break"
 
+    eol = r"(\r\n)|\n|\r"  # \r\n (Windows), \n (UNIX), or \r (Mac)
+    eol_re = re.compile(eol)
+    eol_convention = os.linesep # Default
+
     def loadfile(self, filename):
         try:
             # open the file in binary mode so that we can handle
@@ -191,8 +195,10 @@ class IOBinding:
 
         chars = self.decode(chars)
         # We now convert all end-of-lines to '\n's
-        eol = r"(\r\n)|\n|\r"  # \r\n (Windows), \n (UNIX), or \r (Mac)
-        chars = re.compile( eol ).sub( r"\n", chars )
+        firsteol = self.eol_re.search(chars)
+        if firsteol:
+            self.eol_convention = firsteol.group(0)
+            chars = self.eol_re.sub(r"\n", chars)
 
         self.text.delete("1.0", "end")
         self.set_filename(None)
@@ -306,8 +312,10 @@ class IOBinding:
     def writefile(self, filename):
         self.fixlastline()
         chars = self.encode(self.text.get("1.0", "end-1c"))
+        if self.eol_convention != "\n":
+            chars = chars.replace("\n", self.eol_convention)
         try:
-            f = open(filename, "w")
+            f = open(filename, "wb")
             f.write(chars)
             f.close()
             return True