Python: Update Local Variable in a Parallel Process from Parent Program

Question

I am relatively new to programming, and what I am asking might be a task that is not possible. What I want to do is start a parallel process, which will continuously run until

Answer 1

Nothing wrong with a queue here, but it's probably more idiomatic to use "shared memory" instead. Less overhead. Here's an example self-contained program:

import time

def loop(i):
    while 1:
        print i.value
        i.value += 1
        time.sleep(1)

if __name__ == "__main__":
    from multiprocessing import Process, Value

    i = Value("i", 1)  # "i" for integer, initial value 1
    p = Process(target=loop, args=(i,))
    p.start()
    for base in range(100, 600, 100):
        time.sleep(2)
        i.value = base

That will probably ;-) display:

1
2
100
101
200
201
300
301
400
401
500
501
502
503
504
505
506
...

But caution: in return for being speedier, this is also more brittle. The kinds of data you can share this way are basically inherited from the C language, and aren't generally as rich as Python data types. In the example above, the type code "i" means we're sharing a signed C int, which is usually a 32-bit signed integer. If, for example, i.value reaches 2147483647, and we add 1 to it, it will suddenly become -2147483648 (yup, negative!). Python ints are unbounded, but C ints aren't.