"""Module for wrappers that add extra functionality to encoders."""
from __future__ import annotations
import copy
import pandas as pd
from sklearn.base import BaseEstimator, TransformerMixin
from sklearn.model_selection import StratifiedKFold
import category_encoders as encoders
from category_encoders import utils
[docs]
class PolynomialWrapper(BaseEstimator, TransformerMixin):
"""Extend supervised encoders to n-class labels, where n >= 2.
The label can be numerical (e.g.: 0, 1, 2, 3,...,n), string or categorical (pandas.Categorical).
The label is first encoded into n-1 binary columns. Subsequently, the inner supervised encoder
is executed for each binarized label.
The names of the encoded features are suffixed with underscore and the corresponding class name
(edge scenarios like 'dog'+'cat_frog' vs. 'dog_cat'+'frog' are not currently handled).
The implementation is experimental and the API may change in the future.
The order of the returned features may change in the future.
Parameters
----------
feature_encoder: Object
an instance of a supervised encoder.
Example
-------
>>> from category_encoders import *
>>> import pandas as pd
>>> from sklearn.datasets import fetch_openml
>>> from category_encoders.wrapper import PolynomialWrapper
>>> display_cols = [
... 'Id',
... 'MSSubClass',
... 'MSZoning',
... 'LotFrontage',
... 'YearBuilt',
... 'Heating',
... 'CentralAir',
... ]
>>> bunch = fetch_openml(name='house_prices', as_frame=True)
>>> # need more than one column
>>> y = bunch.target.map(lambda x: int(min([x, 300000]) / 50000))
>>> X = pd.DataFrame(bunch.data, columns=bunch.feature_names)[display_cols]
>>> enc = TargetEncoder(cols=['CentralAir', 'Heating'])
>>> wrapper = PolynomialWrapper(enc)
>>> encoded = wrapper.fit_transform(X, y)
>>> print(encoded.info())
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 1460 entries, 0 to 1459
Data columns (total 17 columns):
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 Id 1460 non-null float64
1 MSSubClass 1460 non-null float64
2 MSZoning 1460 non-null object
3 LotFrontage 1201 non-null float64
4 YearBuilt 1460 non-null float64
5 CentralAir_3 1460 non-null float64
6 Heating_3 1460 non-null float64
7 CentralAir_2 1460 non-null float64
8 Heating_2 1460 non-null float64
9 CentralAir_5 1460 non-null float64
10 Heating_5 1460 non-null float64
11 CentralAir_6 1460 non-null float64
12 Heating_6 1460 non-null float64
13 CentralAir_1 1460 non-null float64
14 Heating_1 1460 non-null float64
15 CentralAir_0 1460 non-null float64
16 Heating_0 1460 non-null float64
dtypes: float64(16), object(1)
memory usage: 194.0+ KB
None
"""
def __init__(self, feature_encoder: utils.BaseEncoder):
"""Init the polynomial wrapper."""
self.feature_encoder: utils.BaseEncoder = feature_encoder
self.feature_encoders: dict[str, utils.BaseEncoder] = {}
self.label_encoder: encoders.OneHotEncoder | None = None
[docs]
def fit(self, X, y, **kwargs):
"""Fit a multi-label encoder."""
# unite the input into pandas types
X, y = utils.convert_inputs(X, y)
y = pd.DataFrame(y.rename('target'))
# apply one-hot-encoder on the label
self.label_encoder = encoders.OneHotEncoder(
handle_missing='error',
handle_unknown='error',
cols=['target'],
drop_invariant=True,
use_cat_names=True,
)
labels = self.label_encoder.fit_transform(y)
labels.columns = [column[7:] for column in labels.columns]
labels = labels.iloc[:, 1:] # drop one label
# train the feature encoders, it is important to reset feature encoders first
self.feature_encoders = {}
for class_name, label in labels.items():
self.feature_encoders[class_name] = copy.deepcopy(self.feature_encoder).fit(X, label)
[docs]
class NestedCVWrapper(BaseEstimator, TransformerMixin):
"""Cross validate supervised encoders to avoid overfitting.
For a validation or a test set, supervised encoders can be used as follows:
X_train_encoded = encoder.fit_transform(X_train, y_train)
X_valid_encoded = encoder.transform(X_valid)
However, the downstream model will be overfitting to the encoded training data due to
target leakage. Using out-of-fold encodings is an effective way to prevent target leakage.
This is equivalent to:
X_train_encoded = np.zeros(X.shape)
for trn, val in kfold.split(X, y):
encoder.fit(X[trn], y[trn])
X_train_encoded[val] = encoder.transform(X[val])
This can be used in place of the "inner folds" as discussed here:
https://sebastianraschka.com/faq/docs/evaluate-a-model.html
See README.md for a list of supervised encoders.
Discussion: Although leave-one-out encoder internally performs leave-one-out cross-validation,
it is actually the most overfitting supervised model in our library.
To illustrate the issue, let's imagine we have a totally unpredictive nominal feature
and a perfectly balanced binary label. A supervised encoder should encode the feature into a
constant vector as the feature is unpredictive of the label.
But when we use leave-one-out cross-validation, the label ratio cease to be perfectly balanced
and the wrong class label always becomes the majority in the training fold.
Leave-one-out encoder returns a seemingly predictive feature.
And the downstream model starts to overfit to the encoded feature.
Unfortunately, even 10-fold cross-validation is not immune to this effect:
http://www.kdd.org/exploration_files/v12-02-4-UR-Perlich.pdf
To decrease the effect, it is recommended to use a low count of the folds.
And that is the reason why this wrapper uses 5 folds by default.
Based on the empirical results, only LeaveOneOutEncoder benefits greatly from this wrapper.
The remaining encoders can be used without this wrapper.
Parameters
----------
feature_encoder: Object
an instance of a supervised encoder.
cv: int or sklearn cv Object
if an int is given, StratifiedKFold is used by default, where the int
is the number of folds.
shuffle: boolean, optional
whether to shuffle each classes samples before splitting into batches.
Ignored if a CV method is provided.
random_state: int, RandomState instance or None, optional, default=None
if int, random_state is the seed used by the random number generator.
Ignored if a CV method is provided.
Example
-------
>>> import pandas as pd
>>> from category_encoders import *
>>> from category_encoders.wrapper import NestedCVWrapper
>>> from sklearn.datasets import fetch_openml
>>> from sklearn.model_selection import GroupKFold, train_test_split
>>> bunch = fetch_openml(name='house_prices', as_frame=True)
>>> display_cols = [
... 'Id',
... 'MSSubClass',
... 'MSZoning',
... 'LotFrontage',
... 'YearBuilt',
... 'Heating',
... 'CentralAir',
... ]
>>> y = bunch.target > 200000
>>> X = pd.DataFrame(bunch.data, columns=bunch.feature_names)[display_cols]
>>> X_train, X_test, y_train, _ = train_test_split(X, y, random_state=42)
>>> X_train, X_valid, y_train, y_valid = train_test_split(X_train, y_train, random_state=42)
>>> # Define the nested CV encoder for a supervised encoder
>>> enc_nested = NestedCVWrapper(TargetEncoder(cols=['CentralAir', 'Heating']), random_state=42)
>>> # Encode the X data for train, valid & test
>>> X_train_enc, X_valid_enc, X_test_enc = enc_nested.fit_transform(
... X_train, y_train, X_test=(X_valid, X_test)
... )
>>> print(X_train_enc.info())
<class 'pandas.core.frame.DataFrame'>
Int64Index: 821 entries, 1390 to 896
Data columns (total 7 columns):
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 Id 821 non-null float64
1 MSSubClass 821 non-null float64
2 MSZoning 821 non-null object
3 LotFrontage 672 non-null float64
4 YearBuilt 821 non-null float64
5 Heating 821 non-null float64
6 CentralAir 821 non-null float64
dtypes: float64(6), object(1)
memory usage: 51.3+ KB
None
"""
def __init__(self, feature_encoder, cv=5, shuffle=True, random_state=None):
"""Init wrapper."""
self.feature_encoder = feature_encoder
self.__name__ = feature_encoder.__class__.__name__
self.shuffle = shuffle
self.random_state = random_state
if isinstance(cv, int):
self.cv = StratifiedKFold(n_splits=cv, shuffle=shuffle, random_state=random_state)
else:
self.cv = cv
[docs]
def fit(self, X, y, **kwargs):
"""Fit on the base feature_encoder without nested cross validation."""
self.feature_encoder.fit(X, y, **kwargs)