为什么+ =运算符的工作方式与+不同，并为python字典赋值运算符？

Why does += operator work differently than + and assign operator for python dictionary?

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有人能解释一下这种行为吗？

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>>> a = {'hello':'world' , 'good':'food'}
>>> b = [1,2]
>>> b = b + a
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
TypeError: can only concatenate list (not"dict") to list
>>> b += a
>>> b
[1, 2, 'good', 'hello'] <--- Why do the keys get added when dict cannot be added to a list
>>>