Search code examples
pythonprogram-entry-point

Purpose of 'if __name__ == "__main__":'

I am trying to understand some code I found which reads command line arguments (attached below). My concern is what purpose of the "if __name__ == __main__"line is...

Why would I use that line instead of just using the code below, main(sys.argv[1:]). What extra use does it provide?

import sys, getopt

def main(argv):
   inputfile = ''
   outputfile = ''
   try:
      opts, args = getopt.getopt(argv,"hi:o:",["ifile=","ofile="])
   except getopt.GetoptError:
      print 'test.py -i <inputfile> -o <outputfile>'
      sys.exit(2)
   for opt, arg in opts:
      if opt == '-h':
         print 'test.py -i <inputfile> -o <outputfile>'
         sys.exit()
      elif opt in ("-i", "--ifile"):
         inputfile = arg
      elif opt in ("-o", "--ofile"):
         outputfile = arg
   print 'Input file is "', inputfile
   print 'Output file is "', outputfile

if __name__ == "__main__":
   main(sys.argv[1:])
Solution

  • Well, imagine that someone else wants to use the functions in your module in their own program. They import your module... and it starts doing its own thing!

    With the if __name__ == "__main__", this doesn't happen. Your module only "does its thing" if it's run as the main module. Otherwise it behaves like a library. It encourages code reuse by making it easier.

    (As Sheng mentions, you may want to import the module into another script yourself for testing purposes.)