Creating a Custom Data Transformer using Scikit-Learn

June 28, 2021

In machine learning, a data transformer is used to make a dataset fit for the training process. Scikit-Learn enables quick experimentation to achieve quality results with minimal time spent on implementing data pipelines involving preprocessing, machine learning algorithms, evaluation, and inference.


Scikit-Learn provides built-in methods for data preparation before the data is fed into a training model. However, as a data scientist, you may need to perform more custom cleanup processes or adding more attributes that may improve your model’s performance. To do that, you will need to create a custom transformer for your data.

In this article, we will look at how to do that.


To follow along with this tutorial, you should have:

  1. A good understanding of the Python programming language.
  2. Familiarity with the Numpy and Pandas libraries.
  3. A basic knowledge in using Jupyter Notebooks or any other notebook-based technology, e.g., Google Colab.
  4. Python and the libraries mentioned above installed.

Let’s jump into it.

The code snippets are tailored for a notebook, but you can also use regular python files.

Getting Started

Loading the data

We will get our dataset from this repository using the following script:

import os
import tarfile
from six.moves import urllib

OUR_PATH = "datasets/housing"
OUR_DATA_URL = OUR_ROOT_URL + OUR_PATH + "/housing.tgz"

def get_data(our_data_url=OUR_DATA_URL, our_path=OUR_PATH):
      if not os.path.isdir(our_path):
      #setting the zip file path      
      zipfile_path = os.path.join(our_path, "housing.tgz")
      #getting the file from the url and extracting it
      urllib.request.urlretrieve(our_data_url, zipfile_path)
      our_zip_file =


The code is for downloading the data from the URL to not dwell on it. First, we imported the os module for interacting with the Operating System. After that, we imported the tarfile module for accessing and manipulating tar files. Lastly, we imported the urllib for using URL manipulation functions.

Then, we set our paths appropriately. In the get_data() function, we made a directory for our data, retrieved it from the URL then extracted and stored it.

So, in your working directory, you will notice a directory called datasets created. On opening it, you will get another directory called housing with a file named housing.csv in it. We will use this file.

We call the function. Then, we will load the CSV file:

import pandas as pd

def load_our_data(our_path=OUR_PATH):
    #setting the csv file path
    our_file_path = os.path.join(our_path, "housing.csv")
    #reading it using Pandas
    return pd.read_csv(our_file_path)

our_dataset = load_our_data()

First, we imported the pandas library, which loads the CSV data from the specified path, our_file_path.

You can view the data by:


Cleaning the data

The cleaning operation we will do here is filling empty numeric attributes with their median values. We will use the SimpleImputer, an estimator, to do that. But, first, we set the strategy to median to calculate the median value for each column’s empty data.

from sklearn.impute import SimpleImputer
'''setting the `strategy` to `median` so that it calculates the median value for each column's empty data'''
imputer = SimpleImputer(strategy="median")
#removing the ocean_proximity attribute for it is textual
our_dataset_num = our_dataset.drop("ocean_proximity", axis=1)
#estimation using the fit method
#transforming using the learnedparameters
X = imputer.transform(our_dataset_num)
#setting the transformed dataset to a DataFrame
our_dataset_numeric = pd.DataFrame(X, columns=our_dataset_num.columns)

We dropped the ocean_proximity attribute because it’s a text attribute that will handle in the next section.

The result produced is an array, so we converted it to a DataFrame.

Handling text and categorical attributes

We cannot handle text and numerical attributes similarly. So, for example, we cannot compute the median of text.

We will use a transformer for this called the OrdinalEncoder. It is chosen because it is more pipeline friendly. Moreover, it assigns numbers to the corresponding text attributes, e.g., 1 for NEAR and 2 for FAR.

from sklearn.preprocessing import OrdinalEncoder
#selecting the textual attribute
our_text_cats = our_dataset[['ocean_proximity']]
our_encoder = OrdinalEncoder()
#transforming it
our_encoded_dataset = our_encoder.fit_transform(our_text_cats)

Our Data Transformer

This is where we will create the custom transformer. We will be adding these three attributes:

  • Rooms per household.
  • Population per household.
  • Bedrooms per household.

For our transformer to work smoothly with Scikit-Learn, we should have three methods:

  1. fit()
  2. transform()
  3. fit_transform

We include the three methods because Scikit-Learn is based on duck-typing. A class is also used because that makes it easier to include all the methods.

The last one is gotten automatically by using the TransformerMixin as a base class. The BaseEstimator lets us get the set_params() and get_params() methods that are helpful in hyperparameter tuning.

We get the three extra attributes in the transform() method by dividing appropriate attributes. An example would be the following: To get the rooms per household, we divide the number of rooms by the number of households.

import numpy as np
from sklearn.base import BaseEstimator, TransformerMixin
#initialising column numbers
rooms,  bedrooms, population, household = 3,4,5,6

class CustomTransformer(BaseEstimator, TransformerMixin):
    #the constructor
    '''setting the add_bedrooms_per_room to True helps us check if the hyperparameter is useful'''
    def __init__(self, add_bedrooms_per_room = True):
        self.add_bedrooms_per_room = add_bedrooms_per_room
    #estimator method
    def fit(self, X, y = None):
        return self
    def transform(self, X, y = None):
        #getting the three extra attributes by dividing appropriate attributes
        rooms_per_household = X[:, rooms] / X[:, household]
        population_per_household = X[:, population] / X[:, household]
        if self.add_bedrooms_per_room:
            bedrooms_per_room = X[:, bedrooms] / X[:, rooms]
            return np.c_[X, rooms_per_household, population_per_household, bedrooms_per_room]
            return np.c_[X, rooms_per_household, population_per_household]

attrib_adder = CustomTransformer()
our_extra_attributes = attrib_adder.transform(our_dataset.values)            

Our pipeline

We implement them in a pipeline for the data transformation steps to be executed in the correct order:

from sklearn.pipeline import Pipeline
from sklearn.compose import ColumnTransformer
#the numeric attributes transformation pipeline
numeric_pipeline = Pipeline([
        ('imputer', SimpleImputer(strategy="median")),
        ('attribs_adder', CustomTransformer()),
numeric_attribs = list(our_dataset_numeric)
#the textual transformation pipeline
text_attribs = ["ocean_proximity"]
#setting the order of the two pipelines
our_full_pipeline = ColumnTransformer([
        ("numeric", numeric_pipeline, numeric_attribs),
        ("text", OrdinalEncoder(), text_attribs),
'''Finally, scaling the data and learning the scaled parameters from the pipeline
our_dataset_prepared = full_pipeline.fit_transform(our_dataset)

The ColumnTransformer is used to transform columns separately and combine the features produced by each transformer to form a single feature space. The code can be run on Google Colab here.


We have seen the various steps for getting the data, transforming it, and then implementing all the steps in a pipeline. So I hope you got some insights.

Peer Review Contributions by: Lalithnarayan C

About the author

Terrence Aluda

Terrence Aluda is an undergraduate Computer Technology student at the Jomo Kenyatta University of Agriculture and Technology, Kenya skilled in backend web development. His current main area of focus is Data Science. He has a great passion for Artificial Intelligence.

His website is found here.

This article was contributed by a student member of Section's Engineering Education Program. Please report any errors or innaccuracies to