Lecture 42


MCS 275 Spring 2022
David Dumas

Lecture 42: argparse

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Command line interface

In most settings where programs are developed, basic familiarity and comfort with working in a shell/terminal is important.

This is especially true in Unix/Linux, and a lot of computing involves Unix/Linux in some way.

Today we'll focus on Python scripts that are meant to be run and used entirely in a shell, i.e. that use a command line interface or CLI.

Executable Python scripts

        # This example works on most Linux
        """Show Python version and exit"""
        import sys

and then marking the file as executable, using shell command

chmod +x myscript.py

Executable Python scripts

In Unix/Linux you can make a Python script file directly executable by adding an interpreter specification line (starting #!) at the beginning of the file

        #!/usr/bin/env python3
        # This example works on MacOS and most Linux
        """Show Python version and exit"""
        import sys

and then marking the file as executable, using shell command

chmod +x myscript.py

Options and arguments

CLI programs often want to accept:

  • Required positional arguments (e.g. input filename, directory to search, ...)
  • Options (e.g. iterate 5 times, write to "out.txt" instead of terminal, use alternate scrape URL, ...)
  • Flags (e.g. enable verbose output, allow overwriting an existing file, ...)


A configurable aspect of the program's operation that can be set or changed by adding command line argument(s).

E.g. A scraper might default to waiting 30 seconds between requests, but allow you to change this on the command line. Some popular syntaxes:

        scrape --delay 5  # my favorite; human readable!
        scrape -d5        # terse but ok
        scrape -d 5       # also used
        scrape --delay=5  # also used
        scrape -delay 5   # less common
        scrape /d 5       # rare except in Windows
        scrape /delay 5   # rare except in Windows

Linux/MacOS examples:

        # positional argument
        cat mcs275/slides/lecture42.html
        ls mcs275/public/samplecode
        cp lecture_template.html lecture43.html
        # flags: turn feature on or off
        ls -l
        ls --human-readable
        # options
        find . -name '*.html' # recursive search for HTML files

Usage and help

If invalid or insufficient arguments are given, a good CLI program will display a short usage message (explaining how to use it).

It is best to also offer a help flag (e.g. --help or -h) that prints a more detailed usage message and list of options.


Parsing and extracting options, arguments, and flags from sys.argv is difficult to do well.

But in Python you can (and should) usually avoid writing command line parsers from scratch.

The standard library module argparse is flexible and easy to use.

Key features

  • Argument and option type checking
  • Automatic help and usage messages
  • Automatic error messages
  • Allows an option to have both short and long names (e.g. -h and --help)
  • Supports many common ways of writing options

Minimal argparse example from the module docs:

    import argparse
    parser = argparse.ArgumentParser()
        help="display a square of a given number",
        type=int  # if not specified, default type is string
    args = parser.parse_args()  # parse or show error and exit
    print(args.square**2) # arguments and options are attributes of
                          # the `args` object returned above


Revision history

  • 2022-04-25 Initial publication