crap, here is the attachment.
Yeah, but I really would like to use regex in the C version (unless you strongly disadvise), so we will have the same logic and the same problem. And I never made a patch for the C interpreter itself, so the C equivalent is not close to be here soon. (btw if you have a starting point to recommend)

I definitely do not like this fix, it destroys the elegance and the simplicity of the "single-dispatch" solution. And it introduce a lot of noisy code for a very rare case, people subclassing datetime.* classes.

Maybe making the regex dictionary having string keys instead of class and passing the correct string from the calling function, like:

    def fromisoformat(string):
        _parse_isodatetime('time', string)

or maybe functools.singledispatch handle this case ?