Function parse_interpolation 

pub(crate) fn parse_interpolation(
    scheme: InterpolationArg,
    tension: Option<f64>,
    end_slopes: Option<(f64, f64)>,
) -> PyResult<Interpolation>

The interpolation scheme a caller gave, with the parameters the scheme carrying them needs.

Named as a string, a scheme takes exactly the keywords it uses: supplying tension to a cubic or omitting it from a tension spline is the caller’s error, not a default to be filled in silently. Given as a value, the scheme carries its own parameters, and a keyword beside it is refused rather than preferred either way.