Introduction to Python's argparse library

Being comfortable using the command line is essential for any software developer, especially when building tools and utilities that need to be run repeatedly, automated, and/or integrated into larger systems.

While graphical interfaces can be helpful, they’re not always practical, and sometimes they’re hard to develop for internal tooling. Instead, we can resort to the command line and leverage a built-in package: argparse.

It’s a Python library that:

• Accepts and parses command-line arguments.

• Automatically generates help messages (-h / --help).

• Converts arguments into Python variables for you.

» argparse does rival click, which is an alternative package for command line application creation. Check out the differences here.

Building a command line application using argparse is straight-forward:

import argparse

# Create parser
parser = argparse.ArgumentParser(
    description="Area Calculator"
)

# Add arguments (positional)
parser.add_argument(
    "length", type=float, help="Length in cm"
)
parser.add_argument(
    "width", type=float, help="Width in cm"
)

# Parse arguments
args = parser.parse_args()

# Perform the calculations
area = args.length * args.width
print(f"The area is: {area} cm²")

First, we’ll instantiate an argparse.ArgumentParser class, which allows us to add arguments (add_argument) and also parse them (parse_args).

Here, we’re adding 2 arguments: length and width. These correlate to where we’ll read them in on the command line. then we’ll parse them using parse_args, calculate the area, and print it.

In the command line, you’ll run this script using the command:

[user]$ python area.py 5.0 3.2
# Output: The area is: 16.0 cm²

One advantage of the package is that it comes built-in with a help menu. To do so, pass in the -h flag:

[user]$ python area.py -h

'''
Output:
Area Calculator

positional arguments:
  a           Length in cm
  b           Width in cm
'''

Understanding the parameters in add_argument

In our example above, we have the following line:

parser.add_argument(
    "width", type=float, help="Width in cm"
)

The first parameter is the name of the argument - here, it’s width. This will be used as a variable after parsing and for users to see when they pull up the help menu.

The second command is type. It’s important to know that argparse by default will be a string, unless this parameter is set. You can pass in int, float, str, bool, complex or pass any callable/custom function that does the conversion.

The third command is help, which is used to show what belongs in the help menu.

Other keyword arguments

Here are some of the keyword arguments you can use with the add_argument function:

Summary

argparse allows you to pass input via the CLI and convert it into your desirable data type. With argparse, you can:

• View the help message with ‘-h’

• Pass any callable or custom function in type which converts a string into any type

• Leverage variety of built-in parameters like default, action, choices to make your code readable.

Final thoughts: Along with being a beginner-friendly way to build command-line interfaces in Python, argparse also validates input, handles types, and generates helpful usage messages automatically, making it a solid, production-ready solution.

Happy coding!

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