该*指示的位置参数的结尾。此后的每个参数只能由关键字指定。这在PEP 3102中定义
>>> def foo1(a, b=None):
... print(a, b)
...
>>> def foo2(a, *, b=None):
... print(a, b)
...
>>> foo1(1, 2)
1 2
>>> foo2(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: foo1() takes 1 positional argument but 2 were given
>>> foo2(1, b=2)
1 2
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