Vajrasky: Note that you can reply to individual review comments within Rietveld; this way context is kept and replies are easier :).  The same people get the message either way.

Anyhow, for float point 1: you can use I believe you can use (unaryfunc) for nb_int and nb_float, just like nb_positive above.

list point 2: I agree, it's not worth it to try to not use the impl function.

rlimit: I'm not sure what value to give, and I'm not where I can play with it until my PC catches fire, either.  Is there some value that makes no sense as a legitimate value?  Is it legal for rlim_cur to be greater than rlim_max?  Or is there a value that is just exceedingly uncommon?  Or you could simply pick some random value; your converter should be written to render with self.err_occurred_if("_return_value == RLIMIT_ERROR_VALUE").