--- a/Lib/fileinput.py	Thu Jun 28 01:45:48 2012 +0200
+++ b/Lib/fileinput.py	Thu Jun 28 12:21:26 2012 +0300
@@ -224,6 +224,9 @@
         if mode not in ('r', 'rU', 'U', 'rb'):
             raise ValueError("FileInput opening mode must be one of "
                              "'r', 'rU', 'U' and 'rb'")
+        if 'U' in mode:
+            warnings.warn("Use of 'U' mode is deprecated",
+                          DeprecationWarning)
         self._mode = mode
         if openhook:
             if inplace: