commandline_args.md

Command-line Arguments

One easy way to use your programs more flexibly is through calling them with command-line arguments from a terminal:

Two ways to implement CL arguments are sys.argv and argparse.

Option 1: sys.argv

The sys.argv method is a quick-and-dirty approach. It is quick to implement, but not very clean in the long run.

The list sys.argv contains everything that was entered on the terminal. For instance, if you write

:::bash
python hello.py Kristian

the sys.argv list will contain two strings:

:::python
['hello.py', 'Kristian']

You can access these values in your code like this:

:::python
import sys

if len(sys.argv) == 2:
    name = sys.argv[1]
    print(f'Hello {name}')
else:
    print("usage: hello.py <name>")

Option 2: argparse

This is a cleaner version. argparse allows you to define command-line parameters for a Python application. The module takes care of reading the parameters into variables, checking data types and generating help text.

argparse is installed with Python by default.

Usage

1. Define the argparse options in your code (in a function or your main block).
2. Call args = parser.parse_args()
3. Access your options as attributes of args

In addition to using your option, your program will print usage instructions when you type

:::bash
python myprog.py --help

Example

Here is a hello world program that you can use as a starting point:

import argparse

parser = argparse.ArgumentParser(description='A Hello World program with arguments.')

parser.add_argument('-m', '--message', type=str, default="Hello ",
               help='message to be written')

parser.add_argument('-c', '--capitals', type=bool,
               help='write capitals')

parser.add_argument('name', type=str, nargs='+',
               help='name(s) of the user')

args = parser.parse_args()

message = args.message
if args.name:
    message = message + ' '.join(args.name)
if args.capitals:
    names = names.upper()
print(f"{message} {names}")

Where to learn more?

https://docs.python.org/3/howto/argparse.html