Display help message with Python argparse when script is called without any arguments

Assume I have a program that uses argparse to process command line arguments/options. The following will print the ‘help’ message:

./myprogram -h

or:

./myprogram --help

But, if I run the script without any arguments whatsoever, it doesn’t do anything. What I want it to do is to display the usage message when it is called with no arguments. How is that done?

Answer

This answer comes from Steven Bethard on Google groups. I’m reposting it here to make it easier for people without a Google account to access.

You can override the default behavior of the error method:

import argparse
import sys

class MyParser(argparse.ArgumentParser):
    def error(self, message):
        sys.stderr.write('error: %sn' % message)
        self.print_help()
        sys.exit(2)

parser = MyParser()
parser.add_argument('foo', nargs='+')
args = parser.parse_args()

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Note that the above solution will print the help message whenever the error method is triggered. For example, test.py --blah will print the help message too if --blah isn’t a valid option.

If you want to print the help message only if no arguments are supplied on the command line, then perhaps this is still the easiest way:

import argparse
import sys

parser=argparse.ArgumentParser()
parser.add_argument('foo', nargs='+')
if len(sys.argv)==1:
    parser.print_help(sys.stderr)
    sys.exit(1)
args=parser.parse_args()

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Note that parser.print_help() prints to stdout by default. As init_js suggests, use parser.print_help(sys.stderr) to print to stderr.