def train_test_split_features(data_train, data_test, zone, features):
    """Returns a pd.DataFrame with the explanatory variables and
    a pd.Series with the target variable, for both train and test data.

    Args:
      data_train (pd.DataFrame): Train data set.
      data_tes (pd.DataFrame): Test data set.
      zone (int): The zone id (id of the wind farm).
      features (list): A list of the column names to be used.

    Returns:
      (pd.DataFrame): Explanatory variables of train data set.
      (pd.Series): Target variable fo train data set.
      (pd.DataFrame): Explanatory variables of test data set.
      (pd.Series): Target variable fo test data set.

    """
    X_train = data_train[data_train.ZONEID == zone][features]
    y_train = data_train[data_train.ZONEID == zone].TARGETVAR

    X_test = data_test[data_test.ZONEID == zone][features]
    y_test = data_test[data_test.ZONEID == zone].TARGETVAR
    return X_train, X_test, y_train, y_test