How to implement both positional arguments and flag options in python3 command line with argparse?

Question

I want to implement a python3 command line by argparse, which supports positional arguments and flag options.

For example:

usage: mycmd [-v] [-h] [-o] text

mycmd text       print text
mycmd -h         show help message and exit
mycmd -v         show version info
mycmd -o text    print text and dump text to file mycmd.txt

Here's my implementation:

import argparse

class MyCmd:
  def __init__(self):
    self.parser = argparse.ArgumentParser()
    self.parser.add_argument('text', help='print text')  # mycmd text
    self.parser.add_argument('-v', action='store_true', dest='version', help='show version info') # mycmd -v
    self.parser.add_argument('-o', dest='output', help='print text and dump text to file mycmd.txt')
  def parse(self):
    return self.parser.parse_args()
  def text(self, text):
    print(text)
  def version(self):
    print('version info: mycmd-0.0.1')
  def output(self, text):
    print(text)
    fp = open('mycmd.txt', 'w')
    fp.write(text + '\n')
    fp.close()

if __name__=='__main__':
  mycmd = MyCmd()
  (options, args) = mycmd.parse()
  if options.text is not None:
    mycmd.text(text)
  elif options.version is not None:
    mycmd.version()
  elif options.output is not None:
    mycmd.output(options.output)

When I test it with:

$ ./mycmd -v

Gives me error:

usage: mycmd [-h] [-o OUTPUT]
            [-v]
            text
mycmd: error: the following arguments are required: text

Why mycmd cannot consume mycmd -v ?

Using '-v' does not keep it from trying to parse all arguments, and raising an error when a required one is missing. But there is a 'version' action class that will print a value and exit before doing the rest of the parsing. — hpaulj
– hpaulj, Commented Apr 30, 2020 at 15:41

Moraes · Accepted Answer · 2020-04-30 18:33:45Z

What your original implementation is missing is the step of including a specific subparse to the text processing flow.

Here's a more simplified version (without a class):

import argparse


def version():
    return 'v0.0.1'

def print_and_save(text, file_path):
    print(text)
    if file_path:
        with open(file_path, 'w') as fp:
            fp.write(text + '\n')
            fp.close()


if __name__=='__main__':
    # Root (rt)
    root_parser = argparse.ArgumentParser()
    root_parser.add_argument('-v', '--version', action='store_true', help='show version info')

    root_sbparsers = root_parser.add_subparsers(help='sub-command help')

    # Adding sub-commands
    text_parser = root_sbparsers.add_parser('process', help='print text')
    text_parser.add_argument('text', type=str, help='text you want to work with')
    text_parser.add_argument('-o', type=str, default=None, help='print text and dump text to file mycmd.txt')

    args = root_parser.parse_args()

    if args.version:
        print(version())
    elif args.text:
        print_and_save(args.text, args.o)

I'm not sure why you have to embed it on a whole class but here's the version with it:

import argparse

class MyCmd(object):
    def __init__(self):
        # Root (rt)
        self.parser = argparse.ArgumentParser()
        self.parser.add_argument('-v', '--version', action='store_true', help='show version info')

        root_sbparsers = self.parser.add_subparsers(help='sub-command help')

        # Adding sub-commands
        text_parser = root_sbparsers.add_parser('process', help='print text')
        text_parser.add_argument('text', type=str, help='text you want to work with')
        text_parser.add_argument('-o', type=str, default=None, help='print text and dump text to file mycmd.txt')

    def parse(self):
        return self.parser.parse_args()

    def text(self, text):
        print(text)

    def version(self):
        print('version info: mycmd-0.0.1')

    def output(self, text, file_path):
        with open(file_path, 'w') as fp:
            fp.write(text + '\n')
            fp.close()


if __name__=='__main__':
    mycmd = MyCmd()
    args = mycmd.parse()

    if args.version:
        mycmd.version()
    elif args.text:
        mycmd.text(args.text)
        if args.o:
            mycmd.output(args.text, args.o)

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