python 3 dict函数 神奇的参数规则

>>> dict({1:2},2=3)
SyntaxError: keyword can't be an expression
>>> dict({1:2},**{2:3})
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
dict({1:2},**{2:3})
TypeError: keyword arguments must be strings
>>> dict({1:2},**{'2':3})
{1: 2, '2': 3}
>>> dict({1:2},'2'=3)
SyntaxError: keyword can't be an expression
>>>