$ ./python 
Python 3.2rc1+ (unknown, Jan 18 2011, 00:55:20) 
>>> import _string
>>> _string.formatter_field_name_split(1)
python: Objects/stringlib/string_format.h:1347: formatter_field_name_split: Assertion `((((((PyObject*)(self))->ob_type))->tp_flags & ((1L<<28))) != 0)' failed.
Abandon

$ ./python 
Python 3.2rc1+ (unknown, Jan 18 2011, 00:55:20) 
>>> import _string
>>> _string.formatter_parser(1)
python: Objects/stringlib/string_format.h:1206: formatter_parser: Assertion `((((((PyObject*)(self))->ob_type))->tp_flags & ((1L<<28))) != 0)' failed.
Abandon

Attached patch fixes both crashes and add tests on the _string module (which has only these two functions).