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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def hex_key(num):
    """You have been tasked to write a function that receives a hexadecimal number as a string and counts the number of hexadecimal digits that are primes (prime number, or a prime, is a natural number greater than 1 that is not a product of two smaller natural numbers). Hexadecimal digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. Prime numbers are 2, 3, 5, 7, 11, 13, 17,... B (=decimal 11), D (=decimal 13). and symbols A,B,C,D,E,F are always uppercase. For num = "AB" the output should be 1. For num = "1077E" the output should be 2. For num = "ABED1A33" the output should be 4. For num = "123456789ABCDEF0" the output should be 6. For num = "2020" the output should be 2. """
    primes = ('2', '3', '5', '7', 'B', 'D')
    total = 0
    for i in range(0, len(num)):
        if num[i] in primes:
            total += 1
    return total
