def replace_practice(str,a,b,count=None):
    i=0
    #记录替换count次数
    n=0
    result=[]
    if count==None:
        count=len(str)
    while i<len(str) and len(str)!=0 and n<count:
        if str[i:i+len(a)]==a:
            result+=list(str[0:i])+list(b)
            str=str[i+len(a):]
            n+=1
            i=0
        else:
            i+=1
    result.append(str)
    return ''.join(result)

if __name__ == '__main__':
    string = '** *112567*  *abc** *98765**   **xyz***'
    print(string)
    a = []
    print('替换结果：',replace_practice(string,' ','mm'))