I think you’re looking for something more like the multiprocessing module:
http://docs.python.org/library/multiprocessing.html#the-process-class
The subprocess module is for spawning processes and doing things with their input/output – not for running functions.
Here is a multiprocessing
version of your code:
from multiprocessing import Process, Queue
# must be a global function
def my_function(q, x):
q.put(x + 100)
if __name__ == '__main__':
queue = Queue()
p = Process(target=my_function, args=(queue, 1))
p.start()
p.join() # this blocks until the process terminates
result = queue.get()
print result