diff -r 18d927fb8671 Objects/fileobject.c
--- a/Objects/fileobject.c	Wed Oct 03 13:48:17 2012 +0530
+++ b/Objects/fileobject.c	Thu Oct 04 14:47:33 2012 +0530
@@ -273,6 +273,8 @@
 _PyFile_SanitizeMode(char *mode)
 {
     char *upos;
+    char *s;
+    int rwa = 0, plus = 0, text_mode = 0, binary = 0;
     size_t len = strlen(mode);
 
     if (!len) {
@@ -280,6 +282,62 @@
         return -1;
     }
 
+    s = mode;
+    while (*s) {
+        switch (*s++) {
+        case 'x':
+            if (rwa) {
+            bad_mode:
+                PyErr_SetString(PyExc_ValueError,
+                                "Must have exactly one of create/read/write/append "
+                                "mode and at most one plus");
+                return -1;
+            }
+            rwa = 1;
+            break;
+        case 'r':
+            if (rwa)
+                goto bad_mode;
+            rwa = 1;
+            break;
+        case 'w':
+            if (rwa)
+                goto bad_mode;
+            rwa = 1;
+            break;
+        case 'a':
+            if (rwa)
+                goto bad_mode;
+            rwa = 1;
+            break;
+        case 'b':
+            binary = 1;
+            if (binary && text_mode) {
+            bad_mix_mode:
+                PyErr_SetString(PyExc_ValueError,
+                                "Can't have text and binary mode at once");
+                return -1;
+            }
+            break;
+        case '+':
+            if (plus)
+                goto bad_mode;
+            plus = 1;
+            break;
+        case 'U':
+            break;
+        case 't':
+            text_mode = 1;
+            if (binary && text_mode)
+                goto bad_mix_mode;
+            break;
+        default:
+            PyErr_Format(PyExc_ValueError,
+                         "invalid mode: %.200s", mode);
+            return -1;
+        }
+    }
+
     upos = strchr(mode, 'U');
     if (upos) {
         memmove(upos, upos+1, len-(upos-mode)); /* incl null char */