Postscript: Things are not as clean as I have hoped and said. However, I don't think this changes the general form of the conclusion. Just that a little more work is needed to achieve the goal.

Python 3.7 and 3.8:

>>> def f(x): pass

>>> f()

TypeError: f() missing 1 required positional argument: 'x'

Also note that in Python 3.8 we have:

>>> def f(x,/): pass

>>> f()

TypeError: f() missing 1 required positional argument: 'x'

Recall that we have

>>> len()

TypeError: len() takes exactly one argument (0 given)

Note that the parameter of 'f' has a name, but not the parameter for 'len'. I think I was expecting

>>> def f(x,/): pass

>>> f()

TypeError: f() takes exactly one argument (0 given)

Jonathan