bpo-41373: IDLE: Fix saving files loaded with no newlines or mixed newlines (GH-21597)

Fixes regression in 3.8.4 and 3.9.0b4.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 0dd463c8a4269137ebed7cc29605c555030df94f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index 74ebefd..8bb2fa6 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -155,6 +155,17 @@ class IOBinding:
                                    parent=self.text)
             return False
 
+        if not isinstance(eol_convention, str):
+            # If the file does not contain line separators, it is None.
+            # If the file contains mixed line separators, it is a tuple.
+            if eol_convention is not None:
+                tkMessageBox.showwarning("Mixed Newlines",
+                                         "Mixed newlines detected.\n"
+                                         "The file will be changed on save.",
+                                         parent=self.text)
+                converted = True
+            eol_convention = os.linesep  # default
+
         self.text.delete("1.0", "end")
         self.set_filename(None)
         self.fileencoding = fileencoding