字符串转化为数字

#给定一个字符串表示整数，将它转化为整数并输出
#例如字符串“345”输出整数345
#例如“-123”输出-123
#思路应该这样。既然是整数，要考虑正负，要考虑输入有非法字符，要考虑是否有溢出


input_char = raw_input("input your string")

translateNumber = 0
isAnumber = True
symbol = 'null'
for s in range(len(input_char)):
    if s == 0:
        if input_char[s] == '+':
            symbol = '+'
            continue
        elif input_char[s] == '-':
            symbol = '-'
            continue

    if not input_char[s].isdigit() and s != 0:
        isAnumber = False
        print "the string is not all number"
        break
    else:
        translateNumber *= 10
        translateNumber = translateNumber + int(input_char[s])

if isAnumber == True:
    print symbol,translateNumber

else:
    pass