I want to use the natural cubic smoothing splines smooth.spline
from R in Python (like som many others want as well (Python natural smoothing splines, Is there
This little trick will work around the specific problem you're having, by allowing you to write "lambda" in a string.
kwargs = {"x": r_x, "y": r_y, "lambda": 42}
spline1 = r_smooth_spline(**kwargs)
In the general case, you can pass around argument containers easily with tuples and dicts.
# as normal
f = function("foo", "bar", my_kwarg="my_value")
# the same call using argument containers
args = ("foo", "bar")
kwargs = {"my_kwarg": "my_value"}
f = function(*args, **kwargs)