Sorry, typo. Should have been len(sys.argv) == 1. Full script:

import argparse
import sys

parser = argparse.ArgumentParser()
parser.add_argument('--foo')

if len(sys.argv) == 1:
    parser.print_help()  
else:
    print(parser.parse_args())

With that script, I see:

$ ./python.exe temp.py 
usage: temp.py [-h] [--foo FOO]

optional arguments:
  -h, --help  show this help message and exit
  --foo FOO