Python argparse: Parse Typed Options and Check Invalid Input

Start with a small, explicit task

A fictional summarizer accepts either short or full mode plus an optional count. Parse one valid call, inspect its help and deliberately reject a noninteger count. The count here controls a returned description only; there are no source files or external services.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_argparse.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

import argparse
import contextlib
import io
parser = argparse.ArgumentParser(prog="summarize")
parser.add_argument("mode", choices=["short", "full"])
parser.add_argument("--count", type=int, default=3)
args = parser.parse_args(["short", "--count", "5"])
assert (args.mode, args.count) == ("short", 5)
assert "--count" in parser.format_help()
assert "short" in parser.format_help()
errors = io.StringIO()
with contextlib.redirect_stderr(errors):
    try:
        parser.parse_args(["full", "--count", "many"])
    except SystemExit as exc:
        assert exc.code == 2
    else:
        raise AssertionError("expected parser rejection")
assert "invalid int value" in errors.getvalue()
print(f"mode={args.mode}; count={args.count}")
print("help checked; invalid integer rejected")

Explain the result

The required positional mode must match one of the two choices. --count is optional, and type=int makes the valid text 5 become an integer before application logic uses it. The help check verifies that the interface exposes those options without fixing the entire help layout to one exact formatting version.

The malformed count is rejected with exit status 2 through SystemExit. The script captures stderr locally and checks the relevant error phrase. A normal command-line program would usually let argparse display that message and exit; catching it here makes the failure path part of a repeatable teaching exercise.

Avoid these mistakes

Type conversion is not complete business validation. An integer count can still be negative or too large for your task, so enforce a stated range separately when required. For a real command-line entry point, parse_args() without the explicit fixture list reads the command’s arguments; retain explicit lists in focused parser tests.