Base 16 vers base 10

tHexa = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f']
print("entrez un entier naturel")
unHexa = (input("saisie de l'Hexadécimal à convert1ir "))
k = 0
unEntier = 0
while len(unHexa) > 0:
    s = chr(unHexa, sys.getsizeof(unHexa)-1,1)
    position = 0
    while s not in tHexa[position]:
        position = position +1
    unEntier = unEntier + position * 16**k
    unHexa = chr(unHexa, 0,sys.getsizeof(unHexa)-1)
    k = k + 1
    print (unEntier)