为何exec()结合Tkinter的END使用时出现报错？

问题背景 #

我编写了两段Python代码：

1. 第一段是包含Tkinter组件的代码（保存为文件）：

from tkinter import *
from tkinter.filedialog import askopenfilename, asksaveasfilename
x = ""
win = Tk()
win.title("TEST")
b = ""
e = Text(width = 40)
e.pack()
def but ():
    global x
    global e
    global b
    global by
    e.config(state = "disabled")
    x = e.get("1.0", END)
    win.destroy()
by = Button(width = 30, height = 10, text = "Publish", command = but)
by.pack()
win.mainloop()
print("")
print(x)

2. 第二段代码通过exec()读取并执行上述文件：

from tkinter.filedialog import askopenfilename, asksaveasfilename
def open_file():
    filepath = askopenfilename(
        filetypes=[("Text Files", "*.txt"), ("All Files", "*.*")]
    )
    if not filepath:
        return
    with open(filepath, mode="r", encoding="utf-8") as input_file:
        text = input_file.read()
        exec(text)
open_file()

点击按钮触发but函数时，出现如下错误：

Exception in Tkinter callback
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/tkinter/__init__.py", line 1921, in __call__
    return self.func(*args)
  File "<string>", line 15, in but
NameError: name 'END' is not defined

问题原因 #

当用exec()执行文件内容时，from tkinter import *导入的名称（包括END）仅存在于open_file函数的局部命名空间中。而Tkinter的回调函数but运行时，会在模块级全局命名空间查找END，导致找不到该名称，触发NameError。

解决办法 #

方案1：让exec使用模块全局命名空间 #

修改exec()的调用，指定全局命名空间为模块的globals()，这样from tkinter import *导入的名称会被放到模块全局，回调函数就能找到END：

from tkinter.filedialog import askopenfilename, asksaveasfilename
def open_file():
    filepath = askopenfilename(
        filetypes=[("Text Files", "*.txt"), ("All Files", "*.*")]
    )
    if not filepath:
        return
    with open(filepath, mode="r", encoding="utf-8") as input_file:
        text = input_file.read()
        exec(text, globals())  # 指定全局命名空间
open_file()

方案2：在被执行的代码中显式引用tkinter模块 #

避免使用from tkinter import *，改为导入tkinter模块并通过模块名访问END，这样不受命名空间影响：

import tkinter as tk
from tkinter.filedialog import askopenfilename, asksaveasfilename
x = ""
win = tk.Tk()
win.title("TEST")
b = ""
e = tk.Text(width=40)
e.pack()
def but():
    global x
    global e
    global b
    global by
    e.config(state="disabled")
    x = e.get("1.0", tk.END)  # 使用tk.END
    win.destroy()
by = tk.Button(width=30, height=10, text="Publish", command=but)
by.pack()
win.mainloop()
print("")
print(x)

方案3：在回调函数内部导入END #

在but函数内部添加from tkinter import END，确保函数执行时能获取到该名称：

from tkinter import *
from tkinter.filedialog import askopenfilename, asksaveasfilename
x = ""
win = Tk()
win.title("TEST")
b = ""
e = Text(width=40)
e.pack()
def but():
    from tkinter import END  # 内部导入END
    global x
    global e
    global b
    global by
    e.config(state="disabled")
    x = e.get("1.0", END)
    win.destroy()
by = Button(width=30, height=10, text="Publish", command=but)
by.pack()
win.mainloop()
print("")
print(x)

内容的提问来源于stack exchange，提问作者Xavion Mirchandani