为什么Tkinter小部件存储为None？（AttributeError：“ NoneType”

Question

#AttributeError: 'NoneType' object has no attribute ... Example

try:                        # In order to be able to import tkinter for

    import tkinter as tk    # either in python 2 or in python 3

except ImportError:

    import Tkinter as tk

root = tk.Tk()

widget = tk.Label(root, text="Label 1").grid()

widget.config(text="Label A")

root.mainloop()

上面的代码产生错误：

Traceback (most recent call last):

  File "C:\Users\user\Documents\Python\other\script.py", line 8, in <module>

    widget.config(text="Label A")

AttributeError: 'NoneType' object has no attribute 'config'

类似地，代码片段：

#TypeError: 'NoneType' object does not support item assignment Example

try:                        # In order to be able to import tkinter for

    import tkinter as tk    # either in python 2 or in python 3

except ImportError:

    import Tkinter as tk

root = tk.Tk()

widget = tk.Button(root, text="Quit").pack()

widget['command'] = root.destroy

root.mainloop()

产生错误：

Traceback (most recent call last):

  File "C:\Users\user\Documents\Python\other\script2.py", line 8, in <module>

    widget['command'] = root.destroy

TypeError: 'NoneType' object does not support item assignment

在两种情况下：

>>>print(widget)

None

为什么这样，为什么widget存储为None，或者为什么在尝试配置窗口小部件时出现上述错误？

这个问题是基于此，并问了广义回答很多关于这个主题相关的和重复的问题。参见此以拒绝编辑。