fix format spec recursive expansion (closes python#19729)

Author: benjaminp

Lib/test/test_unicode.py

@@ -955,6 +955,7 @@ def __format__(self, format_spec):
                          '')
 
         self.assertEqual("{[{}]}".format({"{}": 5}), "5")
+        self.assertEqual("0x{:0{:d}X}".format(0x0,16), "0x0000000000000000")
 
     def test_format_map(self):
         self.assertEqual(''.format_map({}), '')

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.3.4 release candidate 1?
 Core and Builtins
 -----------------
 
+- Issue #19729: In str.format(), fix recursive expansion in format spec.
+
 - Issue #19638: Fix possible crash / undefined behaviour from huge (more than 2
   billion characters) input strings in _Py_dg_strtod.
 

Objects/stringlib/unicode_format.h

@@ -727,8 +727,10 @@ MarkupIterator_next(MarkupIterator *self, SubString *literal,
     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;
+            if (!hit_format_spec) {
+                count = 1;
+                hit_format_spec = 1;
+            }
             break;
         case '{':
             /* the format spec needs to be recursively expanded.