python 实现字符串转整型

def str2Int(s):
    l=list(s)
    if len(l)<=0:
        return 0
    flag=0
    sum=0
    dict_num={'':0,'':1,'':2,'':3,'':4,'':5,'':6,'':7,'':8,'':9}
    dict_tag={'+':1,'-':-1}
    if not dict_tag.__contains__(l[0]) and not dict_num.__contains__(l[0]):
        return 0
    elif dict_tag.__contains__(l[0]):
        flag=dict_tag[l[0]]
    else:
        sum+=dict_num[l[0]]
    for i in range(1,len(l)):
        if l[i]<'' or l[i]>'':
            return 0
        else:
            sum=sum*10+dict_num[l[i]]
    if flag==-1:
        return -sum
    else:
        return sum