如何返回任意两个字母之间的最大距离（az 忽略大小写）？

Question

I'm trying to return a function which takes a string as input and returns the biggest distance between any two letters (az ignoring case). Distance in this case means the number of letters apart in the alphabet. Eg Distance between "a" and "b" is 1. Should I be using string.index or chr and ord ?

My code so far:

def biggest_letter_distance(string):
    dist = [chr(i) for i in range(ord('a'),ord('z')+1)]
    return dist    

What I'm trying to test on:

assert(biggest_letter_distance("Hello World") == 19)
assert(biggest_letter_distance("!!") == -1)

Answer 1

def biggest_letter_distance(string):
    string = [char.lower() for char in string if char.isalpha()]
    if not string:
        return -1
    letters = sorted(set(string))
    return ord(letters[-1]) - ord(letters[0])

Explaining:

1. Selects only the alphabetic characters and turns them to lowercase.
2. Orders the selection
3. Returns the distance between the first and last items