Spline Support like Patsy

[petrhrobar]

Would it be possible to also implement (apart from already implemented interactions and category variable) splines feature:

Patsy library does this really nicely.

Example

[matthewwardrop]

Hi @petrhrobar ,

Formulaic already supports poly (orthogonal polynomial) and bs (basis splines) bases. Which additional spline bases do you need?

[petrhrobar]

Hey There,

COOL!. I was not aware of this. Is there any form of documentation covering the poly and bsfeatures?

[matthewwardrop]

Online documentation is a weak point... but we'll get there. Currently the only documentation is the grammar documentation, which indicates that support exists: https://matthewwardrop.github.io/formulaic/concepts/grammar/#transforms.

The API for these transforms is the same as patsy/R, and the inline documentation is thorough (e.g. https://github.com/matthewwardrop/formulaic/blob/main/formulaic/transforms/basis_spline.py#L37). The website documentation needs to be updated, but I already know this so I'll close this out for now :).

[petrhrobar]

Thanks for that.

Just a follow-up - how to change degrees of freedom in string formula:

import seaborn as sns
from formulaic import Formula, formula
df = sns.load_dataset("tips")


Formula("total_bill ~ bs(size, df = 10)").get_model_matrix(df)

If I call the function itself, it works. however, having it changeable in the string is what I would like:

from formulaic.transforms import basis_spline, poly
pd.DataFrame(basis_spline(df['size'], degree = 10)).values

[matthewwardrop]

Hi again!

The bs function in the formula is literally basis_spline... so you would pass the arguments through directly. For example:

from formulaic.transforms import basis_spline, poly
pd.DataFrame(basis_spline(df['size'], degree = 10)).values

would be achieved using:

from formulaic import Formula
Formula("bs(size, degree = 10)").get_model_matrix(df)

Does this not work for you?

[matthewwardrop]

Ah... I see. There's a bug in the stateful evaluation that attempts to treat the 10 constant as a column to be evaluated. I'll resolve that ahead of the next release!

[matthewwardrop]

The issue above was due to a bug / issue in astor <0.8. I bumped the dependency, and things should work now on the master branch; soon to be 0.3.4.