最近在看asyncio的代码，看到一个新的用法，查阅完文档之后，发现，在定义函数的参 数列表中，* 后的参数，调用者必须以 key=value 形式调用。因为平时看到的都 是 *args, **kwargs 形式的代码。

.. code:: python

def _make_ssl_transport(self, rawsock, protocol, sslcontext, waiter=None,
                        *, server_side=False, server_hostname=None,
                        extra=None, server=None):
    """Create SSL transport."""
    raise NotImplementedError

错误示范::

$ cat test.py
def foo(this, *, loop):
    print("<foo> been called")

foo(None, loop=None)
foo(None, None)
$ python test.py
<foo> been called
Traceback (most recent call last):
File "test.py", line 6, in <module>
    foo(None, None)
TypeError: foo() takes 1 positional argument but 2 were given

.. [#] http://stackoverflow.com/questions/14301967/python-bare-asterisk-in-function-argument

.. [#] https://docs.python.org/3/reference/compound_stmts.html#function-definitions