API Reference

class feature_engine.encoding.OneHotEncoder(top_categories=None, drop_last=False, drop_last_binary=False, variables=None, ignore_format=False)[source]

One hot encoding consists in replacing the categorical variable by a combination of binary variables which take value 0 or 1, to indicate if a certain category is present in an observation. The binary variables are also known as dummy variables.

For example, from the categorical variable “Gender” with categories “female” and “male”, we can generate the boolean variable “female”, which takes 1 if the observation is female or 0 otherwise. We can also generate the variable “male”, which takes 1 if the observation is “male” and 0 otherwise.

The encoder can create k binary variables per categorical variable, k being the number of unique categories, or alternatively k-1 to avoid redundant information. This behaviour can be specified using the parameter drop_last.

The encoder has the additional option to generate binary variables only for the top n most popular categories, that is, the categories that are shared by the majority of the observations in the dataset. This behaviour can be specified with the parameter top_categories.


Only when creating binary variables for all categories of the variable, we can specify if we want to encode into k or k-1 binary variables, where k is the number if unique categories. If we encode only the top n most popular categories, the encoder will create only n binary variables per categorical variable. Observations that do not show any of these popular categories, will have 0 in all the binary variables.

The encoder will encode only categorical variables by default (type ‘object’ or ‘categorical’). You can pass a list of variables to encode. Alternatively, the encoder will find and encode all categorical variables (type ‘object’ or ‘categorical’).

With ignore_format=True you have the option to encode numerical variables as well. The procedure is identical, you can either enter the list of variables to encode, or the transformer will automatically select all variables.

The encoder first finds the categories to be encoded for each variable (fit). The encoder then creates one dummy variable per category for each variable (transform).


New categories in the data to transform, that is, those that did not appear in the training set, will be ignored (no binary variable will be created for them). This means that observations with categories not present in the train set, will be encoded as 0 in all the binary variables.

Also Note

The original categorical variables are removed from the returned dataset when we apply the transform() method. In their place, the binary variables are returned.

top_categories: int, default=None

If None, a dummy variable will be created for each category of the variable. Alternatively, we can indicate in top_categories the number of most frequent categories to encode. In this case, dummy variables will be created only for those popular categories and the rest will be ignored, i.e., they will show the value 0 in all the binary variables.

drop_last: boolean, default=False

Only used if top_categories = None. It indicates whether to create dummy variables for all the categories (k dummies), or if set to True, it will ignore the last binary variable and return k-1 dummies.

drop_last_binary: boolean, default=False

Whether to return 1 or 2 dummy variables for binary categorical variables. When a categorical variable has only 2 categories, then the second dummy variable created by one hot encoding can be completely redundant. Setting this parameter to True, will ensure that for every binary variable in the dataset, only 1 dummy is created.

variables: list, default=None

The list of categorical variables that will be encoded. If None, the encoder will find and transform all variables of type object or categorical by default. You can also make the transformer accept numerical variables, see the next parameter.

ignore_format: bool, default=False

Whether the format in which the categorical variables are cast should be ignored. If false, the encoder will automatically select variables of type object or categorical, or check that the variables entered by the user are of type object or categorical. If True, the encoder will select all variables or accept all variables entered by the user, including those cast as numeric.



Dictionary with the categories for which dummy variables will be created.


The group of variables that will be transformed.


A list with binary variables identified from the data. That is, variables with only 2 categories.


The number of features in the train set used in fit.


If the variables are intended for linear models, it is recommended to encode into k-1 or top categories. If the variables are intended for tree based algorithms, it is recommended to encode into k or top n categories. If feature selection will be performed, then also encode into k or top n categories. Linear models evaluate all features during fit, while tree based models and many feature selection algorithms evaluate variables or groups of variables separately. Thus, if encoding into k-1, the last variable / category will not be examined.


One hot encoding of top categories was described in the following article:


Niculescu-Mizil, et al. “Winning the KDD Cup Orange Challenge with Ensemble Selection”. JMLR: Workshop and Conference Proceedings 7: 23-34. KDD 2009



Learn the unique categories per variable


Replace the categorical variables by the binary variables.


Fit to the data, then transform it.

fit(X, y=None)[source]

Learns the unique categories per variable. If top_categories is indicated, it will learn the most popular categories. Alternatively, it learns all unique categories per variable.

X: pandas dataframe of shape = [n_samples, n_features]

The training input samples. Can be the entire dataframe, not just seleted variables.

y: pandas series, default=None

Target. It is not needed in this encoded. You can pass y or None.

  • If the input is not a Pandas DataFrame.

  • f user enters non-categorical variables (unless ignore_format is True)

  • If there are no categorical variables in the df or the df is empty

  • If the variable(s) contain null values


inverse_transform is not implemented for this transformer.


Replaces the categorical variables by the binary variables.

X: pandas dataframe of shape = [n_samples, n_features]

The data to transform.

X: pandas dataframe.

The transformed dataframe. The shape of the dataframe will be different from the original as it includes the dummy variables in place of the of the original categorical ones.


DataFrame ..


If the input is not a Pandas DataFrame

  • If the variable(s) contain null values.

  • If dataframe has different number of features than the df used in fit()


The OneHotEncoder() replaces categorical variables by a set of binary variables, one per unique category. The encoder has the option to create k or k-1 binary variables, where k is the number of unique categories.

The encoder can also create binary variables for the n most popular categories, n being determined by the user. This means, if we encode the 6 more popular categories, we will only create binary variables for those categories, and the rest will be dropped.

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split

from feature_engine.encoding import OneHotEncoder

# Load dataset
def load_titanic():
        data = pd.read_csv('')
        data = data.replace('?', np.nan)
        data['cabin'] = data['cabin'].astype(str).str[0]
        data['pclass'] = data['pclass'].astype('O')
        data['embarked'].fillna('C', inplace=True)
        return data

data = load_titanic()

# Separate into train and test sets
X_train, X_test, y_train, y_test = train_test_split(
                        data.drop(['survived', 'name', 'ticket'], axis=1),
                        data['survived'], test_size=0.3, random_state=0)

# set up the encoder
encoder = OneHotEncoder( top_categories=2, variables=['pclass', 'cabin', 'embarked'], drop_last=False)

# fit the encoder

# transform the data
train_t= encoder.transform(X_train)
test_t= encoder.transform(X_test)

{'pclass': [3, 1], 'cabin': ['n', 'C'], 'embarked': ['S', 'C']}