easygui课后作业第四题疑问

爱吃肥肉的马

import easygui as g
import os

file_path = g.fileopenbox(default="*.txt")

with open(file_path) as old_file:
    title = os.path.basename(file_path)
    msg = "文件【%s】的内容如下：" % title
    text = old_file.read()
    text_after = g.textbox(msg, title, text)
   
if text != text_after[:-1]:
    # textbox 的返回值会追加一个换行符
    choice = g.buttonbox("检测到文件内容发生改变，请选择以下操作：", "警告", ("覆盖保存", "放弃保存", "另存为..."))
    if choice == "覆盖保存":
        with open(file_path, "w") as old_file:
            old_file.write(text_after[:-1])
    if choice == "放弃保存":
        pass
    if choice == "另存为...":
        another_path = g.filesavebox(default=".txt")
        if os.path.splitext(another_path)[1] != '.txt':
            another_path += '.txt'
        with open(another_path, "w") as new_file:
            new_file.write(text_after[:-1])


为什么打开文件后，点cancel，会报TypeError？
Traceback (most recent call last):
  File "C:/Users/Tina Hu/Desktop/1.py", line 12, in <module>
    if text != text_after[:-1]:
TypeError: 'NoneType' object is not subscriptable

逃兵

text_after =  g.textbox(msg, title, text)
点击cancel相当于关闭了g.textbox(msg, title, text)
变量text_after就会变为None
在执行text != text_after[:-1]语句时，由于text_after为None，所以不能切片
可以外层嵌套个if来解决

import easygui as g
import os

file_path = g.fileopenbox(default="*.txt")
if file_path:
    with open(file_path) as old_file:
        title = os.path.basename(file_path)
        msg = "文件【%s】的内容如下：" % title
        text = old_file.read()
        text_after = g.textbox(msg, title, text)

    if text_after:
        if text != text_after[:-1]:
            # textbox 的返回值会追加一个换行符
            choice = g.buttonbox("检测到文件内容发生改变，请选择以下操作：", "警告", ("覆盖保存", "放弃保存", "另存为..."))
            if choice == "覆盖保存":
                with open(file_path, "w") as old_file:
                    old_file.write(text_after[:-1])
            if choice == "放弃保存":
                pass
            if choice == "另存为...":
                another_path = g.filesavebox(default=".txt")
                if os.path.splitext(another_path)[1] != '.txt':
                    another_path += '.txt'
                with open(another_path, "w") as new_file:
                    new_file.write(text_after[:-1])