Python HTML实体加密问题

下面的代码对在代码底部输入的文件进行加密。注释应解释代码的作用。每次运行代码时，文件都会被值

shift

覆盖。有人知道我如何达到评论中描述的预期结果吗

import random
import html

#Defines text map for characters to traverse.
text_map = []
for i in range(9000):
    text_map.append(html.unescape("&#" + str(i) + ";"))

#Defines shift amount for each individual character in file.
shift = random.randint(2, len(text_map))

#Encription code:
if input("Do you want to encrypt a file? [y/n] ") == "y":

    #Clears final encrypted file.
    encrypted_file = []

    #Encrypt function.
    def encrypt(file_name):

        #Read all lines of file.
        file_lines = open(file_name, "r").readlines()

        #For each line in the file...
        for line in file_lines:

            #For each character of each line in the file...
            for character in line:

                #Defines current position of character being encrypted.
                curr_char = character

                #Shifts every individual character in file by a certain amount.
                for _ in range(shift):

                    #Handles list traversal overlap. (Character at end of list? => Bring to beginning of list.)
                    if curr_char == text_map[-1]:
                        curr_char = text_map[0]
                    else:
                        curr_char = text_map[text_map.index(curr_char) + 1]

                #Appends each shifted character to a new file.
                encrypted_file.append(curr_char)

        #Cleans new encrypted file and overwrites the original file.
        open(file_name, "w").write("".join(encrypted_file))
        open(file_name, "a").write(str(shift))
        print("File encrypted.")

    #Calls encrypt function based upon user input.
    encrypt(input("Enter file name to encrypt: "))