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Did you see my class attributes suggestion a couple messages back? That might solve your dispatch problem:
def _parse_isodatetime(cls, string):
 match = cls._iso_regex.match(...)
class time:
 _iso_regex = re.compile(...)
# or
time._iso_regex = re.compile(...)
date._iso_regex = ...
For the C module, start looking at /Modules/datetimemodule.c. Maybe if you can think of a similar function implemented in C to copy off. It looks like strptime() defers to a Python-only implementation; maybe that is another option.