Akira, your patch does this:

-        self._writetranslate = newline != ''
-        self._writenl = newline or os.linesep
+        self._writetranslate = newline in (None, '\r', '\r\n')
+        self._writenl = newline if newline is not None else os.linesep

Any reason you made the second change? Why change the value assigned to _writenl for newline='\n' when you don't want to actually change the behavior for those cases? Just so you can double-check at write time that _writetranslate is never set unless _writenl is '\r', '\r\n', or os.linesep?