The del statement makes the variable local, as alluded to by

http://docs.python.org/dev/3.0/reference/simple_stmts.html#the-del-
statement

The manual is clearer about assignments, which are local unless declared 
global or nonlocal.


For other question, me thinks you need to write slightly cleverer code 
that passes only required data.  I have experience with 
multiprocessing.Pool().map which lets me control the arguments I pass to 
functions.