class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        LOOKUP = {
            'A': 1,             'B': 2,
            'C': 3,             'D': 4,
            'E': 5,             'F': 6,
            'G': 7,             'H': 8,
            'I': 9,             'J': 10,
            'K': 11,            'L': 12,
            'M': 13,            'N': 14,
            'O': 15,            'P': 16,
            'Q': 17,            'R': 18,
            'S': 19,            'T': 20,
            'U': 21,            'V': 22,
            'W': 23,            'X': 24,
            'Y': 25,            'Z': 26
        }
        
        value = 0
        
        for index, letter in enumerate(reversed(s)):
            value += LOOKUP[letter] * 26 ** index
            
        return value

class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        LOOKUP = {
            'A': 1,             'B': 2,
            'C': 3,             'D': 4,
            'E': 5,             'F': 6,
            'G': 7,             'H': 8,
            'I': 9,             'J': 10,
            'K': 11,            'L': 12,
            'M': 13,            'N': 14,
            'O': 15,            'P': 16,
            'Q': 17,            'R': 18,
            'S': 19,            'T': 20,
            'U': 21,            'V': 22,
            'W': 23,            'X': 24,
            'Y': 25,            'Z': 26,
        }
        
        value = 0
        
        for index, letter in enumerate(reversed(s)):
            value += LOOKUP[letter] * 26 ** index
            
        return value

Update solution.py