Issue 6089: str.format raises SystemError.

Lib/test/test_str.py

@@ -351,6 +351,10 @@ class StrTest(
         self.assertRaises(IndexError, "{:s}".format)
         self.assertRaises(IndexError, "{}".format)
 
+        # issue 6089
+        self.assertRaises(ValueError, "{0[0]x}".format, [None])
+        self.assertRaises(ValueError, "{0[0](10)}".format, [None])
+
         # can't have a replacement on the field name portion
         self.assertRaises(TypeError, '{0[{1}]}'.format, 'abcdefg', 4)
 

Lib/test/test_unicode.py

@@ -1100,6 +1100,10 @@ class UnicodeTest(
         self.assertRaises(IndexError, u"{:s}".format)
         self.assertRaises(IndexError, u"{}".format)
 
+        # issue 6089
+        self.assertRaises(ValueError, u"{0[0]x}".format, [None])
+        self.assertRaises(ValueError, u"{0[0](10)}".format, [None])
+
         # can't have a replacement on the field name portion
         self.assertRaises(TypeError, u'{0[{1}]}'.format, u'abcdefg', 4)
 

Objects/stringlib/string_format.h

@@ -375,8 +375,9 @@ FieldNameIterator_next(FieldNameIterator *self, int *is_attribute,
         *name_idx = get_integer(name);
         break;
     default:
-        /* interal error, can't get here */
-        assert(0);
+        /* Invalid character follows ']' */
+        PyErr_SetString(PyExc_ValueError, "Only '.' or '[' may "
+                        "follow ']' in format field specifier");
         return 0;
     }