Command-line interface

1. Execute
2. Parse arguments
   1. Define argument parser

Execute

What	How	Details
Start Python script, wait for completion	os.system('python script.py')
Start Python script with arguments, wait for completion	os.system('python script.py -file "hello.csv"')

Parse arguments

Uses the argparse module.

What	How	Details
Test if no arguments were provided	len(sys.argv) <= 1
Number of arguments	len(sys.argv)
Get program or script that invoked the process	sys.argv[0]

Define argument parser

Requires a parser instance to be created first.

import argparse
parser = argparse.ArgumentParser(
  prog = 'My CLI program',
  description = 'Description of functionality here',
  epilog = 'Text at the bottom of the help file'
)

What	How	Details
Define positional argument	parser.add_argument('file')
Define optional positional argument	parser.add_argument('dest', nargs='?')
Define positional argument accepting multiple values	parser.add_argument('files', type = str, nargs='+')
Define flag to enable a feature (default is False)	parser.add_argument('-f', '--force', action = 'store_true')
Define flag to disable a feature (default is True)	parser.add_argument('--disable', action = 'store_false')
Define string option	parser.add_argument('-s', '--source', type = str)
Define int option	parser.add_argument('--seed', type = int)
Define float option	parser.add_argument('value', type = float)
Define categorical option	parser.add_argument('answer', choices = ['a', 'b', 'c'])
Define valid path option	parser.add_argument('path', type = pathlib.Path)
Define openable file option	parser.add_argument('file', type = open)
Define writable file option	parser.add_argument('dest', type = argparse.FileType('w'))
Define option that optionally takes a value	parser.add_argument('--seed', nargs = '?')
Define option that takes a value	parser.add_argument('--seed')
Define required option	parser.add_argument('--seed', required = True)	Required options are considered bad form. Preferably use positional arguments instead.

Then parse arguments using:

args = parser.parse_args()