Using Arguments with argparse

diagram what argparse does before your code runs mermaid

  flowchart TD
  A["sys.argv"] --> B["parser.parse_args()"]
  B --> C{"a required positional missing?"}
  C -->|yes| D["print usage, SystemExit(2)"]
  C -->|no| E{"does each value fit its type=?"}
  E -->|no| D
  E -->|yes| F["build a Namespace"]
  F --> G["--dry-run becomes ns.dry_run"]
  F --> H["-n '3' becomes the int 3"]
  G --> I["your script runs, with values already validated"]
  H --> I
  J["-h / --help"] --> K["print help, SystemExit(0)"]

The parser is not just a way to read sys.argv -- it validates, converts and, when something is wrong, exits. That last part surprises people: a bad value does not raise an exception you can catch in the ordinary way, it prints usage and terminates with status 2.

python

parser.add_argument("--dry-run", action="store_true")
ns = parser.parse_args(["/tmp", "--dry-run", "-n", "3"])
# Namespace(path='/tmp', dry_run=True, n=3)

The dash becomes an underscore. --dry-run is read as ns.dry_run. Writing ns.dry-run is a syntax error and getattr(ns, "dry-run") returns nothing useful, so the flag appears to be ignored.

A bad value exits the process. -n abc does not raise ValueError — argparse prints the usage line and raises SystemExit(2). That is right for a command-line tool and wrong inside a library or a test, where you should catch SystemExit or use parse_known_args.

Why argparse

It gives your scripts:

--help
typed arguments
clear usage patterns

Example

argparse_example.py

import argparse
 
 
def build_parser():
    p = argparse.ArgumentParser(description="Example automation CLI")
    p.add_argument("--dry-run", action="store_true", help="Don’t change files")
    p.add_argument("--source", required=True, help="Source folder")
    return p
 
 
def main():
    args = build_parser().parse_args()
    print("source:", args.source)
    print("dry_run:", args.dry_run)
 
 
if __name__ == "__main__":
    main()

Using Arguments with argparse

Why argparse

Example

🧪 Try It Yourself

Exercise 1 – List Files with os.listdir

Exercise 2 – Join Paths with os.path.join

Exercise 3 – Write and Read a File

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