I'm developing a pipeline to fit parameters for a gradient boosting classifier while also fitting the optimum number of features in a PCA model. This is the current setup:

pipe = Pipeline([
    ('reduce_dim', PCA()),
    ('classify', GradientBoostingClassifier())
score = {'f1': 'f1', 'accuracy': 'accuracy'}
N_FEATURES_OPTIONS = [12,13,14,15]
max_dep = [3,4,5,6]
n_est = [50,80,100, 120, 150]
min_samp = [4,5,6,10]
param_grid = [
        'reduce_dim': [PCA()],
        'reduce_dim__n_components': N_FEATURES_OPTIONS,
        'classify__n_estimators': n_est,
        'classify__max_depth': max_dep,
reducer_labels = ['PCA']

grid_adc = GridSearchCV(pipe, cv=5, n_jobs=-1, param_grid=param_grid, scoring=score, refit='accuracy')
grid_adc.fit(X_train, y_train)

Which outputs:

{'classify__max_depth': 3,
 'classify__min_samples_split': 4,
 'classify__n_estimators': 50,
 'reduce_dim': PCA(copy=True, iterated_power='auto', n_components=12, random_state=None,
   svd_solver='auto', tol=0.0, whiten=False),
 'reduce_dim__n_components': 12}

Now I'd like to validate and score the model with cross validation. If I run the following:

cross_val_score(grid, X_train, y_train, cv=5, n_jobs=-1)

Does the PCA I fit from the pipeline carry over to the cross_val_score function? If so, does the cross_val_score function transform the data with the PCA every time it generates a new train/test split?

Or do I need to create a new PCA after the pipeline to fit into the cross_val_score function?

  • $\begingroup$ In cross_val_score(grid, X_train, y_train, cv=5, n_jobs=-1) should you change grid for grid_adc? $\endgroup$ Commented May 31, 2018 at 10:01
  • $\begingroup$ Correct, that's a typo. I was cobbling together a few versions of this for the question. $\endgroup$
    – Karl
    Commented May 31, 2018 at 15:19

1 Answer 1


When doing GridSearchCv, the best model is already scored. You can access it with the attribute best_score_ and get the model with best_estimator_. You do not need to re-score it in a cross validation.

Also, yes, the pipeline is entirely fitted when doing each split during the cv.


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