plpython implementation

I'm reading through plperl and plpython implementations and I don't
understand the way they work.

Comments for plperl say that there are two interpreters (trusted and
untrusted) for each user session, and they are stored in a hash.

Plpython version looks quite different, there is no such global hash with
interpreters, there is just a pointer to an interpreter and one global
function _PG_init, which runs once (but per session, user, or what?).

I'm just wondering how a plpython implementation should look like. We need
another interpreter, but PG_init function is run once, should it then
create two interpreters on init, or should we let this function do nothing
and create a proper interpreter in the first call of plpython(u) function
for current session?

thanks,
Szymon