>>> def foo(a):
...     pass
... 
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() missing 1 required positional argument: 'a'

This error is incorrect. It says "positional argument", but it's just "argument". The proof is that if you call it with

foo(a=3)

it works fine.