Mask a card number

easypaymentsPython

Support staff can see the last four digits of a card and nothing else.

Every character except the last four becomes an asterisk.
Anything four characters or shorter comes back untouched — there is nothing safe to hide.

mask_card(card_no: string) → string

Where you start

def mask_card(card_no: str) -> str:

Build the asterisks from the length, then glue on the tail.

Reference solution in Python

def mask_card(card_no: str) -> str:
    if len(card_no) <= 4:
        return card_no
    return '*' * (len(card_no) - 4) + card_no[-4:]