25. Command-Line Arguments

The argparse module helps you build command-line programs that accept flags and parameters. It is the standard-library way to turn a script into a proper CLI tool.

25.1. Basic arguments

Create a parser, declare the arguments you expect, and then read them from args.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('name')
parser.add_argument('age', type=int)
args = parser.parse_args()

print(f'{args.name} is {args.age} years old.')

25.2. Flags

Boolean flags are a good fit for optional behavior such as --verbose.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--verbose', action='store_true')
args = parser.parse_args()

if args.verbose:
    print('verbose mode enabled')
else:
    print('verbose mode disabled')

25.3. Choices

You can restrict an argument to a small set of valid values.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('mode', choices=['fast', 'safe', 'debug'])
args = parser.parse_args()

print(f'mode={args.mode}')

25.4. Exercise

Write a command-line temperature converter that accepts a numeric value and a mode of either c-to-f or f-to-c. Print the converted result.

25.4.1. Solution

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('value', type=float)
parser.add_argument('mode', choices=['c-to-f', 'f-to-c'])
args = parser.parse_args()

if args.mode == 'c-to-f':
    result = args.value * 9 / 5 + 32
else:
    result = (args.value - 32) * 5 / 9

print(result)