python_取出字符串中出现2次的字符串

#取出字符串中出现2次的字符串，使用count方法统计
def two_zifuchuan(str):
    s=set()
    for i in str:
        if str.count(i)==2:
            s.add(i)
    return s

#取出字符串中出现2次的字符串，使用字典统计
def two_occur(str):
    s={}
    for i in str:
        if i in s.keys():
            s[i]+=1
        else:
            s[i]=1
    return [i for i in s if s[i]==2]

str="dddredddddewws22dff43"
print(two_zifuchuan(str))
print(two_occur(str))