sklearn.LabelEncoder with never seen before values

Question

If a sklearn.LabelEncoder has been fitted on a training set, it might break if it encounters new values when used on a test set.

The only solution I c

Answer 1

I have created a class to support this. If you have a new label comes, this will assign it as unknown class.

from sklearn.preprocessing import LabelEncoder
import numpy as np


class LabelEncoderExt(object):
    def __init__(self):
        """
        It differs from LabelEncoder by handling new classes and providing a value for it [Unknown]
        Unknown will be added in fit and transform will take care of new item. It gives unknown class id
        """
        self.label_encoder = LabelEncoder()
        # self.classes_ = self.label_encoder.classes_

    def fit(self, data_list):
        """
        This will fit the encoder for all the unique values and introduce unknown value
        :param data_list: A list of string
        :return: self
        """
        self.label_encoder = self.label_encoder.fit(list(data_list) + ['Unknown'])
        self.classes_ = self.label_encoder.classes_

        return self

    def transform(self, data_list):
        """
        This will transform the data_list to id list where the new values get assigned to Unknown class
        :param data_list:
        :return:
        """
        new_data_list = list(data_list)
        for unique_item in np.unique(data_list):
            if unique_item not in self.label_encoder.classes_:
                new_data_list = ['Unknown' if x==unique_item else x for x in new_data_list]

        return self.label_encoder.transform(new_data_list)

The sample usage:

country_list = ['Argentina', 'Australia', 'Canada', 'France', 'Italy', 'Spain', 'US', 'Canada', 'Argentina, ''US']

label_encoder = LabelEncoderExt()

label_encoder.fit(country_list)
print(label_encoder.classes_) # you can see new class called Unknown
print(label_encoder.transform(country_list))


new_country_list = ['Canada', 'France', 'Italy', 'Spain', 'US', 'India', 'Pakistan', 'South Africa']
print(label_encoder.transform(new_country_list))