Why does pool run the entire file multiple times?

Question

I'm trying to understand the output from this Python 2.7.5 example script:

import time
from multiprocessing import Pool

print(time.strftime('%Y-%m-%d %H:%M', time.localtime(time.time())))
props2=[
            '170339',
            '170357',
            '170345',
            '170346',
            '171232',
            '170363',
            ]
def go(x):
     print(x)

if __name__ == '__main__':
    pool = Pool(processes=3)
    pool.map(go, props2)

print(time.strftime('%Y-%m-%d %H:%M', time.localtime(time.time())))  

This yields the output:

2015-08-06 10:13

2015-08-06 10:13

2015-08-06 10:13

170339

170357

170345

170346

171232

170363

2015-08-06 10:13

2015-08-06 10:13

2015-08-06 10:13

My questions are:

A) Why does the time print three times at the beginning and the end? I would have expected it to print the start time, and then the end time.

B) The real question - How do I get it to run one command multiple times, but all the others a single time?

Answer 1

Python imports the __main__ module for each process. On an import, the whole file is executed again. On python 3, if you remove the if __name__ == '__main__' you will get an infinite loop since the file is getting recursively called.

For the real question:

In python scripts, I typically try to avoid executing any statements or variables on the global scope except for function definitions. I use the below as a template for all python scripts.

import sys

def main(argv):
  #main logic here

if __name__ == '__main__':
  main(sys.argv)

When you have a script with re-usable functions, even if it has a main method, you can import it into another script if you need to.

Answer 2

Multiprocessing needs to import your script in each subprocess in order to use the go() function. When your script is imported, it prints the date. If you only want something to run in the main script, put it in the if __name__ == '__main__' block.