[Chris McDonough]
... self._f[current] = open(self._p[current],'w+b')
.... will be likely to fail at the last line if you're using nonpersistent cache files, because self._p[current] is (bogus) '1-None-0' (relative bogus filename).
Is it really *likely* to fail? It's just a name, and it's opened in 'w' ('+b') mode, not 'r' mode. That is, it creates the file -- no file of that name need already exist (and if one does, it tries to overrwrite it). Running on Windows most days, I'm not usually aware of all the permission bugs Linuxheads delight in torturing themselves with <wink>.
There should probably be a _using_persistent_cache flag attr rather than trying to inspect self._p to find out if we're using persistent caches.
+1. As you later discovered, this "hmm, let's try to guess what we're doing based on obscure droppings" business is a continuing bug factory.
I may try to work up a patch + test for this later.
I'm neutral on whether you try, but +1 on you actually doing it <wink>.