Why is Tkinter widget stored as None? (AttributeError: 'NoneType' object ...)(TypeError: 'NoneType' object ...)

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()

以上代码产生错误：

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'

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类似代码片：

#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

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在这两种情况下：

>>>print(widget)
None

这是为什么，为什么 widget 存储为 None，或者为什么我在尝试配置小部件时出现上述错误？

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此问题基于 this and is asked for a generalized answer to many related and repetitive questions on the subject. See this 编辑拒绝。

Answer 1

widget 存储为 None 因为几何管理器方法 grid, pack, place return None，因此应该在 上调用它们]separate line 而不是创建小部件实例的行，如：

widget = ...
widget.grid(..)

或：

widget = ...
widget.pack(..)

或：

widget = ...
widget.place(..)

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对于问题中的第二个代码片段：

widget = tkinter.Button(...).pack(...)

应该分成两行：

widget = tkinter.Button(...)
widget.pack(...)

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信息： is based on, if not for the most parts copied from, this answer.