python - Luhn's algorithm

app.py

def luhn(card_no):
    card_no = str(card_no)
    pp = "INVALID"
    card_specs = {
        "AMEX": {
            "prefix": ["34", "37"],
            "max_len": [15]
        },
        "VISA": {
            "prefix": ["4"],
            "max_len": [13, 16]
        },
        "MASTERCARD": {
            "prefix": [str(i) for i in range(51, 56)],
            "max_len": [16]
        }
    }
    
    for card, spec in card_specs.items():
        if len(card_no) in spec["max_len"]:
            for prefix in spec["prefix"]:
                if card_no[:len(prefix)] == prefix:
                    pp = card
                    break

    if pp == "INVALID":
        print(pp)
        return

    multip_2 = []
    odd = []
    for i, d in enumerate(card_no[::-1]):
        if (i+1) % 2 == 0:
            multip_2.append(str(int(d) * 2))
        else:
            odd.append(int(d))

    res = sum([int(c) for d in multip_2 for c in d] + odd)
    if res % 10 != 0:
        pp = "INVALID"

    print(pp)