integer to string problem leetcode code example

Example: atoi leetcode

class Solution(object):
    def myAtoi(self, s):
        """
        :type str: str
        :rtype: int
        """
        ###better to do strip before sanity check (although 8ms slower):
        #ls = list(s.strip())
        #if len(ls) == 0 : return 0
        if len(s) == 0 : return 0
        ls = list(s.strip())
        
        sign = -1 if ls[0] == '-' else 1
        if ls[0] in ['-','+'] : del ls[0]
        ret, i = 0, 0
        while i < len(ls) and ls[i].isdigit() :
            ret = ret*10 + ord(ls[i]) - ord('0')
            i += 1
        return max(-2**31, min(sign * ret,2**31-1))