>>> def f(a, b, c):
...     pass
>>> f(a=1, 2, 3)
SyntaxError: positional argument follows keyword argument
>>> f(a=1, *(2, 3))
TypeError: f() got multiple values for argument 'a'


f(a=1, 2, 3) will cause a SyntaxError, but f(a=1, *(2, 3)) will cause a TypeError. This makes me feel confused.

As keyword arguments must follow positional arguments, I suppose a SyntaxError rather than a TypeError should be reported if a variadic argument follows keyword arguments. Would you kindly explain why the CPython takes different actions for these two cases?

See the thread "Order of positional and keyword arguments" on the Python-Dev mailing lists at 2018-04-26:

https://mail.python.org/pipermail/python-dev/2018-April/153157.html

Reproduced in 3.11:

>>> f(a=1, 2, 3)
  File "<stdin>", line 1
    f(a=1, 2, 3)
               ^
SyntaxError: positional argument follows keyword argument
>>> f(a=1, *(2, 3))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() got multiple values for argument 'a'
>>>

This is not a bug, check the thread that Serhiy attached. The grammar is, in principle, correct. For instance, you can do:

def f(a,b,c):

    pass
def wrapper(*args, **kw):
    return f(c=1, *args, **kw)

wrapper(2,b=2)

And you don't want that to be a syntax error.

Also, just to make it clear: changing this would be backwards incompatible, so if someone is interested, a bigger discussion is needed (starting with python-ideas/python-dev).