加密，保存在新的文件里

建立一个文件（c.txt），将以下内容存入到文件中。然后将该文件中的每个英文字母加密后写入到一个新文件（d.txt）。加密的方法是：将A变成B，B变成C，……，Y变成Z，Z变成A；a变成b，b变成c，……，y变成z，z变成a，其他字符不变化。
I say to you today, my friends.
And so even though we face the difficulties of today and tomorrow, I still have a dream. It is a dream deeply rooted in the American dream.

with  open("c.txt", "w") as file:  # w只写方式。如果文件不存在，则新建；存在，则覆盖;a追加方式。如果文件不存在，则新建；存在，则打开继续操作
    file.write("I say to you today, my friends.")  # 写入数据
    file.write("And so even though we face the difficulties of today and tomorrow, I still have a dream. It is a dream deeply rooted in the American dream.") 

with open("d.txt", "w") as fd:
    with open("c.txt") as fc: # 默认“r”
        line = fc.readline()
        words = []
        for letter in line: # 遍历字符
            ascii = ord(letter) # 字符转ascii
            tni = int(ascii) # 小写a97-z122，大写A65-Z90
            if tni >= 97 and tni < 122:
                tni += 1
            elif tni == 122:
                tni = 97
            elif tni >= 65 and tni < 90:
                tni += 1
            elif tni == 90:
                tni = 65
            char = chr(tni) # ascii转字符
            words.append(char)
        encrypted = "".join(words)
        fd.write(encrypted)