How to tell a function to use the default argument values?

One way to do it would be with variadic argument unpacking:

def foo(..., **kwargs):
    ...
    if math.isclose(x, y, **kwargs):
        ...

This would allow you to specify atol and rtol as keyword arguments to the main function foo, which it would then pass on unchanged to math.isclose.

However, I would also say that it is idiomatic that arguments passed to kwargs modify the behaviour of a function in some way other than to be merely passed to sub-functions being called. Therefore, I would suggest that instead, a parameter is named such that it is clear that it will be unpacked and passed unchanged to a sub-function:

def foo(..., isclose_kwargs={}):
    ...
    if math.isclose(x, y, **isclose_kwargs):
        ...

You can see an equivalent pattern in matplotlib (example: plt.subplots – subplot_kw and gridspec_kw, with all other keyword arguments being passed to the Figure constructor as **fig_kw) and seaborn (example: FacetGrid – subplot_kws, gridspec_kws).

This is particularly apparent when there are mutiple sub-functions you might want to pass keyword arguments, but retain the default behaviour otherwise:

def foo(..., f1_kwargs={}, f2_kwargs={}, f3_kwargs={}):
    ...
    f1(**f1_kwargs)
    ...
    f2(**f2_kwargs)
    ...
    f3(**f3_kwargs)
    ...

Caveat:

Note that default arguments are only instantiated once, so you should not modify the empty dicts in your function. If there is a need to, you should instead use None as the default argument and instantiate a new empty dict each time the function is run:

def foo(..., isclose_kwargs=None):
    if isclose_kwargs is None:
        isclose_kwargs = {}
    ...
    if math.isclose(x, y, **isclose_kwargs):
        ...

My preference is to avoid this where you know what you’re doing since it is more brief, and in general I don’t like rebinding variables. However, it is definitely a valid idiom, and it can be safer.