Python dynamic functions with keyword arguments avoid unused parameter

Question

I have a part of code in python, which calls dynamically different functions, where I always want to pass 3 different arguments. However, these functions, might not always need to use those 3 different arguments. Here is a very simple code that shows the issue:

def test_keyword_args():
    def fn1(a, b, c):
        return a + b

    def fn2(a, b, c):
        return a + c

    def fn3(a, b, c):
        return b + c

    obj = {
        'a': fn1,
        'b': fn2,
        'c': fn3,
    }

    for key in obj:
        value = obj[key](a=1, b=2, c=3)
        if key == 'a':
            assert value == 3
        if key == 'b':
            assert value == 4
        if key == 'c':
            assert value == 5

How can I always call same function obj[key](a=1,b=2,c=3) passing this keyword arguments, and avoid complains about unused parameters? (c not used in fn1, b not used in fn2, a not used in fn3)

I can imagine suppressing warnings would do the trick, but I do not think it is the appropriate solution

I am using Python 3.7.3

Answer 1

You can define arguments as keyword only by prefixing the argument list with *, you can then avoid the unused parameter warnings by naming a parameter _. Using **_ allows us to ignore any keyword arguments not in our named parameters

def fn1(*, a, b, **_):
    return a + b

def fn2(*, a, c, **_):
    return a + c

def fn3(*, b, c, **_):
    return b + c