The standard library documentation on module inspect starts with an overview of types and attributes. This overview (in all Python versions) states:
    code.co_names: tuple of names of local variables
    code.co_varnames: tuple of names of arguments and local variables
That suggests the argument names are set(code.co_varnames) - set(code.co_names), which is incorrect.

I think the attribute description should be:
    code.co_names: tuple of names of used global and built-in variables

>>> def f(x): a = 1; print(f, a)
>>> assert f.__code__.co_varnames == ('x', 'a')  # argument and local, in that order
>>> assert set(f.__code__.co_names) == set(('f', 'print'))  # global and built-in (don't care order), not local