从string=“ ”之后我就看不懂了

Question

txt = open("命运.txt", "r", encoding="utf-8").read()
for ch in ' \n':
    txt = txt.replace(ch, "")
d = {}
for ch in txt:
    d[ch] = d.get(ch, 0) + 1
ls = list(d.items())
ls.sort(key=lambda x: x[1], reverse=True)  # 此行可以按照词频由高到低排序
string = ""
for i in range(len(ls)):
    s = str(ls[i]).strip("()")
    if i < len(ls) - 1:
        string = string + s[1] + ':' + s[5:] + ','
    else:
        string = string + s[1] + ':' + s[5:]
f = open("命运-频次排序.txt", "w", encoding="utf-8")
f.write(string)
f.close()