#!/usr/bin/env python3
"""File checksum utility - calculates SHA256 checksum of a file."""
import sys
import hashlib


def sha256_checksum(filepath):
    """Calculate SHA256 checksum of a file, return hex digest."""
    sha256 = hashlib.sha256()
    with open(filepath, 'rb') as f:
        for chunk in iter(lambda: f.read(8192), b''):
            sha256.update(chunk)
    return sha256.hexdigest()


def main():
    if len(sys.argv) != 2:
        print("Usage: checksum.py <filepath>")
        sys.exit(1)
    filepath = sys.argv[1]
    try:
        result = sha256_checksum(filepath)
        print(result)
        return 0
    except FileNotFoundError:
        print(f"Error: File '{filepath}' not found", file=sys.stderr)
        return 1


if __name__ == "__main__":
    sys.exit(main())