A forest counts how much each column improved its splits while it trained. That count is free to read once the forest is fitted.
Task: write top_built_in(n).
load_breast_cancer(return_X_y=True, as_frame=True).train_test_split(X, y, test_size=0.25, random_state=0, stratify=y).RandomForestClassifier(n_estimators=100, random_state=0) on the training rows.Return the names of the n columns with the highest feature_importances_, most important first.