Index: Objects/fileobject.c
===================================================================
--- Objects/fileobject.c	(revision 61453)
+++ Objects/fileobject.c	(working copy)
@@ -155,7 +155,7 @@
 	if (upos) {
 		memmove(upos, upos+1, len-(upos-mode)); /* incl null char */
 
-		if (mode[0] == 'w' || mode[0] == 'a') {
+		if (mode[0] == 'w' || mode[0] == 'a' || strchr(mode, '+')) {
 			PyErr_Format(PyExc_ValueError, "universal newline "
 			             "mode can only be used with modes "
 				     "starting with 'r'");
@@ -2075,7 +2075,7 @@
 "the value for this attribute is one of None (no newline read yet),\n"
 "'\\r', '\\n', '\\r\\n' or a tuple containing all the newline types seen.\n"
 "\n"
-"'U' cannot be combined with 'w' or '+' mode.\n"
+"'U' cannot be combined with 'w', 'a', or '+' mode.\n"
 );
 
 PyTypeObject PyFile_Type = {
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 61453)
+++ Misc/NEWS	(working copy)
@@ -12,6 +12,9 @@
 Core and builtins
 -----------------
 
+- Issue #2091: file() accepted a 'U' mode string containing '+', but 'U' can
+  only be used with 'r'.
+
 - Issue #1779871: Gnu gcc can now build Python on OS X because the
   flags -Wno-long-double, -no-cpp-precomp, and -mno-fused-madd are no
   longer passed.
Index: Lib/test/test_file.py
===================================================================
--- Lib/test/test_file.py	(revision 61453)
+++ Lib/test/test_file.py	(working copy)
@@ -121,7 +121,7 @@
 
     def testModeStrings(self):
         # check invalid mode strings
-        for mode in ("", "aU", "wU+"):
+        for mode in ("", "aU", "wU", "U+", "+U", "rU+"):
             try:
                 f = open(TESTFN, mode)
             except ValueError: