Prove safety: converting a string to an integer

Title Description
Converts a string into an integer, the request can not use the library function converts the integer string. Value of 0 or a character string is not a valid return value 0

Input Description
enter a character string including alphanumeric symbols, can be null

Output Description
If the expression is valid number value is returned, otherwise 0

Example 1
Input
+2147483647
1a33
Output
2147483647
0

Part of the answer, more clever

class Solution:
    def StrToInt(self, s):
        # write code here
        if s is '':
            return 0
        numlist = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']
        res = 0
        label = 1
        first = s[0]
        if first == '+':
            label = 1
            s = s[1:]
        elif first == '-':
            label = -1
            s = s[1:]
        for string in s:
            if string not in numlist:
                return 0
            else:
                res = res * 10 + numlist.index(string)
        return label * res