How to Build an argparse CLI That Filters File Lines by Keyword in Python

import argparse
import sys

def main():
    parser = argparse.ArgumentParser(description="Filter lines from a file by keyword.")
    parser.add_argument("input", type=str, help="File to read")
    parser.add_argument("keyword", type=str, help="Keyword to filter lines")
    parser.add_argument("--contains", action="store_true", help="Keep lines containing keyword (default)")
    parser.add_argument("--not-contains", action="store_true", help="Keep lines NOT containing keyword")
    args = parser.parse_args()

    exclusion_mode = args.not_contains
    if args.contains and args.not_contains:
        print("Error: cannot combine --contains and --not-contains", file=sys.stderr)
        sys.exit(1)

    try:
        with open(args.input, "r") as f:
            lines = [line.rstrip("\n") for line in f]
    except FileNotFoundError:
        print(f"Error: file '{args.input}' not found", file=sys.stderr)
        sys.exit(1)

    filtered = []
    for line in lines:
        is_match = args.keyword in line
        keep = not is_match if exclusion_mode else is_match
        if keep:
            filtered.append(line)

    for line in filtered:
        print(line)

if __name__ == "__main__":
    main()