Getting the leave-one-out error on least square regression to fit polynomials I need to implement least square regression to fit polynomials of degree 1-27. I then need to get the leave-one-out error (kfold cross validation where k = n). After doing a lot of research it seems the best way to get the LOO error is to use sklearn cross_val_score(). My problem is I do not know how or if it is possible to use with regression models. 

n = len(x)
p, res, _, _, _  = numpy.polyfit(x,y,1,full=True)

cv = cross_val_score(?, X, y, scoring=mse, cv=n)



I cannot figure out what the estimator would be or how to make it in cross_val_score. Being new to python and these topics make this twice as challenging.
numpy and scikit-learn are quite separate open-source projects. As such, they are compatible in some ways but not in others. All scikit-learn models are variants on the fundamental estimator object (see https://scikit-learn.org/stable/tutorial/statistical_inference/settings.html). The cross_val_score function expects a scikit-learn-style estimator object and there's no easy way to create such an object from the output of numpy.polyfit.

I'd suggest working entirely in scikit-learn, creating a regression estimator that you can cross-validate with cross_val_score. This SO post shows you how to build that polynomial regression estimator.