Using multiprocessing inside a function - python-3.x

Question

I want to take the working code from below and put it into a function.
import multiprocessing as mp
def parameters(x,n):
for i in x:
yield (i,n)
def power(a):
x, n = a
return x**n
if __name__ == '__main__':
p = [i for i in range(1000)]
p = parameters(p,2)
agents = 4
chunk = 10
with mp.Pool(processes = agents) as pool:
o = pool.map(power,p,chunksize = chunk)
print(o)
So that I can call it whenever I want. I tried doing something like this:
import multiprocessing as mp
def parameters(x,n):
for i in x:
yield (i,n)
def power(a):
x, n = a
return x**n
def calculate(s,n):
p = [i for i in range(s)]
p = parameters(p,n)
agents = 4
chunk = 10
with mp.Pool(processes = agents) as pool:
o = pool.map(power,p,chunksize = chunk)
return o
print(calculate(1000,2))
However this does not work at all, It tells me That another process has started before one has ended. But the code above did work. Is there a way to properly take this code into a function? If not with this setup, then with what setup?

Answer 1

Make sure to protect code that should only run in the main process with if __name__ == '__main__':. This code works:
import multiprocessing as mp
def parameters(x,n):
for i in x:
yield (i,n)
def power(a):
x, n = a
return x**n
def calculate(s,n):
p = [i for i in range(s)]
p = parameters(p,n)
agents = 4
chunk = 10
with mp.Pool(processes = agents) as pool:
o = pool.map(power,p,chunksize = chunk)
return o
if __name__ == '__main__':
print(calculate(1000,2))
Without the if, the following error is raised:
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.
This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:
if __name__ == '__main__':
freeze_support()
...
The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.