what is ,*, ?

[snippsat]

so how can a definition of an argument specifying that it can be given either way define the difference between a required argument and one that is not required (e.g. calling f() above with just one positional  argument)?

It's only forcing that last argument has to be a key word argument.

λ ptpython
>>> def f(positional, either_way_with_default_value='foo',*, only_as_a_named_option='bar'):
...     return positional,either_way_with_default_value,only_as_a_named_option

>>> f(1)
(1, 'foo', 'bar')

>>> f(1, 2)
(1, 2, 'bar')

>>> f(1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() takes from 1 to 2 positional arguments but 3 were given
f() takes from 1 to 2 positional arguments but 3 were given

>>> f(1, 2, only_as_a_named_option=3)
(1, 2, 3)

If remove ,*,.

>>> def f(positional, either_way_with_default_value='foo', only_as_a_named_option='bar'):
...     return positional,either_way_with_default_value,only_as_a_named_option

>>> f(1)
(1, 'foo', 'bar')

>>> f(1, 2)
(1, 2, 'bar')

>>> f(1, 2, 3)
(1, 2, 3)