A simple modification of @conmak's answer to still return a dictionary with None as values when the function has named arguments but no defaults:

def my_fn(a, b=2, c='a'):
    pass

def get_defaults(fn):
    ### No arguments
    if isinstance(fn.__code__.co_varnames, type(None)):
        return {}
    ### no defaults
    if isinstance(fn.__defaults__, type(None)):
        return dict(zip(
            fn.__code__.co_varnames, 
            [None]*(fn.__code__.co_argcount)
        ))
    ### every other case
    return dict(zip(
        fn.__code__.co_varnames, 
        [None]*(fn.__code__.co_argcount - len(fn.__defaults__)) + list(fn.__defaults__)
    ))

print(get_defaults(my_fn))

Should now give:

{'a': None, 'b': 2, 'c': 'a'}