transform_3_sets_missing_values.py

transformed_age = \
pd.DataFrame(imputer_age.transform(train[['Age']]), 
             columns=['Age', 'Age_missing'], 
             index=train.index) # the most important line: do not forget the index

train = train.drop(columns=['Age']).join(transformed_age)

transformed_age = \
pd.DataFrame(imputer_age.transform(validation[['Age']]), 
             columns=['Age', 'Age_missing'], 
             index=validation.index) # the most important line: do not forget the index

validation = validation.drop(columns=['Age']).join(transformed_age)

transformed_age = \
pd.DataFrame(imputer_age.transform(holdout[['Age']]), 
             columns=['Age', 'Age_missing'], 
             index=holdout.index) # the most important line: do not forget the index

holdout = holdout.drop(columns=['Age']).join(transformed_age)