Question

This is probably a trivial question, but how do I parallelize the following loop in python?

# setup output lists
output1 = list()
output2 = list()
output3 = list()

for j in range(0, 10):
    # calc individual parameter value
    parameter = j * offset
    # call the calculation
    out1, out2, out3 = calc_stuff(parameter = parameter)

    # put results into correct output list
    output1.append(out1)
    output2.append(out2)
    output3.append(out3)

I know how to start single threads in Python but I don’t know how to “collect” the results.

Multiple processes would be fine too – whatever is easiest for this case. I’m using currently Linux but the code should run on Windows and Mac as-well.

What’s the easiest way to parallelize this code?

Answer 1

The CPython implementation currently has a global interpreter lock (GIL) that prevents threads of the same interpreter from concurrently executing Python code. This means CPython threads are useful for concurrent I/O-bound workloads, but usually not for CPU-bound workloads. The naming calc_stuff() indicates that your workload is CPU-bound, so you want to use multiple processes here (which is often the better solution for CPU-bound workloads anyway, regardless of the GIL).

There are two easy ways of creating a process pool into the Python standard library. The first one is the multiprocessing module, which can be used like this:

pool = multiprocessing.Pool(4)
out1, out2, out3 = zip(*pool.map(calc_stuff, range(0, 10 * offset, offset)))

Note that this won’t work in the interactive interpreter due to the way multiprocessing is implemented.

The second way to create a process pool is concurrent.futures.ProcessPoolExecutor:

with concurrent.futures.ProcessPoolExecutor() as pool:
    out1, out2, out3 = zip(*pool.map(calc_stuff, range(0, 10 * offset, offset)))

This uses the multiprocessing module under the hood, so it behaves identically to the first version.