用Python实现replace方法

def myReplace(s,sub, dest, times =None):
    #如果times是None，替换的次数是s.count(sub)
    if times == None:
        times = s.count(sub)
    sub_index = []
    sub_length = len(sub)
    dest_length = len(dest)
    s = list(s)
    for i in range(len(s)):
        if s[i:i+sub_length] == list(sub):
            sub_index.append(i)

    n = 0
    for index in sub_index:
        if times > 0:
            offset = n * (dest_length - sub_length)
            index = index + offset
            s[index:index+sub_length] = list(dest)
            n += 1
            times -= 1
    return "".join(s)

print(myReplace("abcc1aha",'a','xy'))
print(myReplace("abcdefgabca","a","123",1))