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bpo-11594: Ensure line-endings are respected when using 2to3 by aaronang · Pull Request #6483 · python/cpython

@aaronang I think I see where things went wrong. In adding newline='' during the read phase, that caused the newlines to be untranslated. But then in the write phase, we left newline=None (the default), which causes \n to be translated to \r\n. As a result, CRLF -> CRCRLF (ref)

I think the patch also needs something like:

cpython master $ git diff
diff --git a/Lib/lib2to3/refactor.py b/Lib/lib2to3/refactor.py
index 7c4e064997..7841b99a5c 100644
--- a/Lib/lib2to3/refactor.py
+++ b/Lib/lib2to3/refactor.py
@@ -514,7 +514,7 @@ class RefactoringTool(object):
         set.
         """
         try:
-            fp = io.open(filename, "w", encoding=encoding)
+            fp = io.open(filename, "w", encoding=encoding, newline='')
         except OSError as err:
             self.log_error("Can't create %s: %s", filename, err)
             return
diff --git a/Lib/lib2to3/tests/test_refactor.py b/Lib/lib2to3/tests/test_refactor.py
index f3059a9311..9e3b8fbb90 100644
--- a/Lib/lib2to3/tests/test_refactor.py
+++ b/Lib/lib2to3/tests/test_refactor.py
@@ -300,6 +300,7 @@ from __future__ import print_function"""
         old, new = self.refactor_file(fn)
         self.assertIn(b"\r\n", old)
         self.assertIn(b"\r\n", new)
+        self.assertNotIn(b"\r\r\n", new)

     def test_refactor_docstring(self):
         rt = self.rt()