diff -r 88a21c5a97ef Lib/test/test_format.py
--- a/Lib/test/test_format.py	Wed Jun 05 00:46:29 2013 +0200
+++ b/Lib/test/test_format.py	Wed Jun 05 01:10:32 2013 +0200
@@ -327,6 +327,23 @@ class FormatTest(unittest.TestCase):
         self.assertIs(text % (), text)
         self.assertIs(text.format(), text)
 
+    def test_precision(self):
+        INT_MAX = 2147483647
+
+        f = 1.2
+        self.assertEqual(format(f, ".0f"), "1")
+        self.assertEqual(format(f, ".3f"), "1.200")
+        with self.assertRaises(ValueError) as cm:
+            format(f, ".%sf" % (INT_MAX + 1))
+        self.assertEqual(str(cm.exception), "precision too big")
+
+        c = complex(f)
+        self.assertEqual(format(f, ".0f"), "1")
+        self.assertEqual(format(f, ".3f"), "1.200")
+        with self.assertRaises(ValueError) as cm:
+            format(f, ".%sf" % (INT_MAX + 1))
+        self.assertEqual(str(cm.exception), "precision too big")
+
 
 def test_main():
     support.run_unittest(FormatTest)
diff -r 88a21c5a97ef Python/formatter_unicode.c
--- a/Python/formatter_unicode.c	Wed Jun 05 00:46:29 2013 +0200
+++ b/Python/formatter_unicode.c	Wed Jun 05 01:10:32 2013 +0200
@@ -982,7 +982,7 @@ format_float_internal(PyObject *value,
     Py_ssize_t n_total;
     int has_decimal;
     double val;
-    Py_ssize_t precision = format->precision;
+    Py_ssize_t precision;
     Py_ssize_t default_precision = 6;
     Py_UCS4 type = format->type;
     int add_pct = 0;
@@ -999,6 +999,12 @@ format_float_internal(PyObject *value,
        from a hard-code pseudo-locale */
     LocaleInfo locale = STATIC_LOCALE_INFO_INIT;
 
+    if (format->precision > INT_MAX) {
+        PyErr_SetString(PyExc_ValueError, "precision too big");
+        goto done;
+    }
+    precision = (int)format->precision;
+
     if (format->alternate)
         flags |= Py_DTSF_ALT;
 
@@ -1132,7 +1138,7 @@ format_complex_internal(PyObject *value,
     Py_ssize_t n_im_total;
     int re_has_decimal;
     int im_has_decimal;
-    Py_ssize_t precision = format->precision;
+    int precision;
     Py_ssize_t default_precision = 6;
     Py_UCS4 type = format->type;
     Py_ssize_t i_re;
@@ -1160,6 +1166,12 @@ format_complex_internal(PyObject *value,
        from a hard-code pseudo-locale */
     LocaleInfo locale = STATIC_LOCALE_INFO_INIT;
 
+    if (format->precision > INT_MAX) {
+        PyErr_SetString(PyExc_ValueError, "precision too big");
+        goto done;
+    }
+    precision = (int)format->precision;
+
     /* Zero padding is not allowed. */
     if (format->fill_char == '0') {
         PyErr_SetString(PyExc_ValueError,