diff -r 60b18d7fbe24 Objects/stringlib/unicode_format.h
--- a/Objects/stringlib/unicode_format.h	Sat Apr 06 16:48:55 2013 +0200
+++ b/Objects/stringlib/unicode_format.h	Sat Apr 06 13:11:15 2013 -0400
@@ -633,17 +633,17 @@ MarkupIterator_init(MarkupIterator *self
 /* returns 0 on error, 1 on non-error termination, and 2 if it got a
    string (or something to be expanded) */
 static int
 MarkupIterator_next(MarkupIterator *self, SubString *literal,
                     int *field_present, SubString *field_name,
                     SubString *format_spec, Py_UCS4 *conversion,
                     int *format_spec_needs_expanding)
 {
-    int at_end;
+    int at_end, hit_format_spec;
     Py_UCS4 c = 0;
     Py_ssize_t start;
     int count;
     Py_ssize_t len;
     int markup_follows = 0;
 
     /* initialize all of the output variables */
     SubString_init(literal, NULL, 0, 0);
@@ -718,22 +718,28 @@ MarkupIterator_next(MarkupIterator *self
        format_specs cannot have braces in them. */
     *field_present = 1;
     count = 1;
 
     start = self->str.start;
 
     /* we know we can't have a zero length string, so don't worry
        about that case */
+    hit_format_spec = 0;
     while (self->str.start < self->str.end) {
         switch (c = PyUnicode_READ_CHAR(self->str.str, self->str.start++)) {
+        case ':':
+            hit_format_spec = 1;
+            count = 1;
+            break;
         case '{':
             /* the format spec needs to be recursively expanded.
                this is an optimization, and not strictly needed */
-            *format_spec_needs_expanding = 1;
+            if (hit_format_spec)
+                *format_spec_needs_expanding = 1;
             count++;
             break;
         case '}':
             count--;
             if (count <= 0) {
                 /* we're done.  parse and get out */
                 SubString s;