Sign a PGP public key using a private key and password, then save the signed key to a file

Question

/IT-Sec$ python sign_key.py 
gpg: key A0E9DEC8202C21F9: "kafbuy <[email protected]>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

Serial number of friend's public key not found.

import subprocess

def sign_public_key(friend_public_key, private_key, passphrase, output_file):
    # Import the friend's public key
    subprocess.run(['gpg', '--import', friend_public_key], check=True)

    # Get the serial number of the friend's public key
    result = subprocess.run(['gpg', '--list-keys', '--with-colons'], stdout=subprocess.PIPE, check=True)
    lines = result.stdout.decode('utf-8').split('\n')
    serial_number = None
    for line in lines:
        if line.startswith('pub:'):
            parts = line.split(':')
            if parts[9] == 'pgp-key-pub':
                serial_number = parts[4]
                break

    if serial_number:
        # Sign the friend's public key
        subprocess.run(['gpg', '--sign-key', serial_number], check=True)

        # Validate the signature
        subprocess.run(['gpg', '--check-sigs', serial_number], check=True)

        # Export the signed public key
        subprocess.run(['gpg', '--armor', '--export', serial_number, '>', output_file], shell=True, check=True)
    else:
        print("Serial number of friend's public key not found.")

# Using the function
sign_public_key('pgp-key-pub.asc', 'my-private-key.asc', 'Jordan', 'signed_friend_public_key.asc')