string to integer | Python Fiddle

class Solution:
    def stringToInt(self, value):
        """
        :type value: an ascii string
        :return type: integer
        """
        #if value contains white space
        value = value.strip()
        res = []
        for i in range(len(value)):
            if i == 0 and value[i] in ('+', '-'):
                res.append(value[i])
            elif "0" <= value[i] <= "9":
                res.append(value[i])
            else:
                return None
        if res in ([], ['+'], ['-']):
            return None
        
        ans = int(''.join(res))
        
        max, min = 2147483647, -2147483648
        if min <= ans <= max: return ans
        elif ans >= max: return max
        else: return min
        
    def getVal(self):
        value = raw_input("Enter a value: ")
        ans = self.stringToInt(value)
        if ans != None:
            print "You entered:" + str(ans)
        else:
            print "You entered an invalid value!"

if __name__ == "__main__":
    s = Solution()
    s.getVal()

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