Optimized Pipeline Detector

In this part, we speculate about possibility of searching for optimized pipelines that perform some preprocessing, sensitivity analysis, and composing estimators to achieve an optimum performance measure, based on a pre-determined set of operators and estimators, all inherited from scikit-learn’s base classes.

Suppose that we have some data transformers \(\mathbb{T}=\{T_1,\dots, T_k\}\) and some estimators \(\mathbb{E}=\{E_1,\dots, E_m\}\) and willing to find a composition \(P=F_1\circ F_2\circ\dots\circ F_n\) where \(F_1\in\mathbb{E}\) and \(F_i\in\mathbb{T}\cup\mathbb{E}\) for \(i=2,\dots,n\) and the composition is optimal with respect to a given performance measure. Each estimator/transformer may accept a number of parameters, discrete or continuous. Note that there are \(m\times(m+k)^{n-1}\) different combinations based on \(\mathbb{T}\) and \(\mathbb{E}\). So, the number of possible pipelines grows exponentially as the number of building blocks increase. Now, if we want to examine all possible combinations of at most \(N\) estimator/transformer, the domain would be of the form

\[\mathbb{U} = \bigcup_{n=1}^N \mathbb{E}\times(\mathbb{E}\cup\mathbb{T})^{n-1}.\]

each element of \(\mathbb{U}\) corresponds to infinitely many functions as the set of acceptable hyperparameters for each one is potentially infinite. Suppose that \(P\in\mathbb{U}\) and \(\tilde{x}\) is the set of its parameters. We use surrogate optimization to find


(or \(\textrm{argmax}\) depending on the task) where \(X\) is the test set, \(\mu\) is the performance and deal with the elements of

\[\tilde{\mathbb{U}}=\{P(\tilde{x}_P) : P\in\mathbb{U}\},\]

that are already optimized wit hrespect to their hyperparameters. This reduces the optimized pipeline detection to searching \(\tilde{\mathbb{U}}\) to find an optimum pipeline. This is still a very heavy task to accomplish given the number of elements in \(\mathbb{U}\) and the computational intensity of a surrogate optimization. Fortunately, the format of the elements of \(\mathbb{U}\) is very much suggestive and demands for a genetic algorithm to reach an optima.

The AML class accepts a set of estimators and transformers, dictionaries of their parameters that can be changed, and searches the space of possible pipelines either exhaustively or according to an evolutionary set up to find an optimum pipeline.

Example 1: The following is a classification based on sk-rebate data:

# Find an optimum classification pipeline

import pandas as pd
import numpy as np
from sklearn.model_selection import RandomizedSearchCV
from sklearn.kernel_ridge import KernelRidge
from sklearn.gaussian_process.kernels import Matern, Sum, ExpSineSquared
from SKSurrogate import *

param_grid_krr = {
    "alpha": np.logspace(-4, 0, 5),
    "kernel": [
        Sum(Matern(), ExpSineSquared(l, p))
        for l in np.logspace(-2, 2, 10)
        for p in np.logspace(0, 2, 10)
regressor = RandomizedSearchCV(
    KernelRidge(), param_distributions=param_grid_krr, n_iter=5, cv=2

config = {
    # Classifiers
    "sklearn.naive_bayes.GaussianNB": {"var_smoothing": Real(1.0e-9, 2.0e-1)},
    "sklearn.linear_model.LogisticRegression": {
        "penalty": Categorical(["l1", "l2"]),
        "C": Real(1.0e-6, 10.0),
        "class_weight": HDReal((1.0e-5, 1.0e-5), (20.0, 20.0))
        # 'dual': Categorical([True, False])
    "sklearn.svm.SVC": {
        "C": Real(1e-6, 20.0),
        "gamma": Real(1e-6, 10.0),
        "tol": Real(1e-6, 10.0),
        "class_weight": HDReal((1.0e-5, 1.0e-5), (20.0, 20.0)),
    "lightgbm.LGBMClassifier": {
        "boosting_type": Categorical(["gbdt", "dart", "goss", "rf"]),
        "num_leaves": Integer(2, 100),
        "learning_rate": Real(1.0e-7, 1.0 - 1.0e-6),  # prior='uniform'),
        "n_estimators": Integer(5, 250),
        "min_split_gain": Real(0.0, 1.0),  # prior='uniform'),
        "subsample": Real(1.0e-6, 1.0),  # prior='uniform'),
        "importance_type": Categorical(["split", "gain"]),
    # Preprocesssors
    "sklearn.preprocessing.StandardScaler": {
        "with_mean": Categorical([True, False]),
        "with_std": Categorical([True, False]),
    "skrebate.ReliefF": {
        "n_features_to_select": Integer(2, 10),
        "n_neighbors": Integer(2, 10),
    # Sensitivity Analysis
    "SKSurrogate.sensapprx.SensAprx": {
        "n_features_to_select": Integer(2, 20),
        "method": Categorical(["sobol", "morris", "delta-mmnt"]),
        "regressor": Categorical([None, regressor]),
import warnings

warnings.filterwarnings("ignore", category=Warning)

genetic_data = pd.read_csv(
X, y = genetic_data.drop("class", axis=1).values, genetic_data["class"].values

A = AML(config=config, length=3, check_point="./", verbose=2)
A.eoa_fit(X, y, max_generation=10, num_parents=10)

In order to perform an exhaustive search on all possible pipelines just replace the last line with the following:

A.fit(X, y)

We can retrieve the top n models via A.get_top(n).

Example 2: The following is a regression based on Airfoil Self-Noise Data Set data:

# Find an optimum regression pipeline

import pandas as pd
import numpy as np
from sklearn.model_selection import RandomizedSearchCV
from sklearn.kernel_ridge import KernelRidge
from sklearn.gaussian_process.kernels import Matern, Sum, ExpSineSquared
from SKSurrogate import *

config = {
    # Regressors
    "sklearn.linear_model.LinearRegression": {"normalize": Categorical([True, False])},
    "sklearn.kernel_ridge.KernelRidge": {
        "alpha": Real(1.0e-4, 10.0),
        "kernel": Categorical(
                Sum(Matern(), ExpSineSquared(l, p))
                for l in np.logspace(-2, 2, 10)
                for p in np.logspace(0, 2, 10)
    # Preprocesssors
    "sklearn.preprocessing.StandardScaler": {
        "with_mean": Categorical([True, False]),
        "with_std": Categorical([True, False]),
    "sklearn.preprocessing.Normalizer": {"norm": Categorical(["l1", "l2", "max"])},
    # Feature Selectors
    "sklearn.feature_selection.VarianceThreshold": {"threshold": Real(0.0, 0.3)},
import warnings

warnings.filterwarnings("ignore", category=Warning)

df = pd.read_csv(
    names=["Frequency", "Angle", "length", "velocity", "thickness", "level"],
X = df.drop("level", axis=1).values
y = df["level"].values

A = AML(
A.eoa_fit(X, y, max_generation=12, num_parents=12)

Some Technical Notes

It should be evident from the example that the config dictionary’s keys could point to any module that is available from the working folder. The only constraint is that the classes being used must inherit from sklearn.base.BaseEstimator, RegressorMixin, ClassifierMixin, TransformerMixin or imblearn.base.SamplerMixin, BaseSampler.

The last estimator will always be selected from either RegressorMixin or ClassifierMixin. The case of imblearn.base.SamplerMixin, BaseSampler can only occur at the beginning of the pipeline. The rest could be RegressorMixin, ClassifierMixin or TransformerMixin.


If a non TransformerMixin occurs in the middle, then by StackingEstimator it will transform the data to append columns based on the outcome of RegressorMixin or ClassifierMixin.

Permutation Importance

If sklearn.pipeline.FeatureUnion is included within the config dictionary, in the scope of a pipeline two scenarios are plausible:

  • `FeatureUnion` is followed by a series of transformations: in this case FeatureUnion
    does exactly what is expected, i.e., gathers all the feature outputs of transformers;
  • `FeatureUnion` is followed by a mixture of transformations and estimators: then
    SKSurrogate uses eli5.sklearn.PermutationImportance to weight the features based on the estimators and AML’s scoring and then selects top features via sklearn.feature_selection.SelectFromModel.

Not all transformers select a subset of of features (e.g., Normalizer or StandardScaler). If FeatureUnion is followed by such transformers, it does not have any effect on the outcome of the transformer. If the transformer selects a subset of features (VarianceThreshold, skrebate.ReliefF) then FeatureUnion collects the outcomes and returns the union. This is also true for PermutationImportance. The FeatureUnion affects the following transformers and estimators until it reaches the last step or a transformer which is not a feature selector. Subclasses of sklearn.feature_selection.base.SelectorMixin are considered as feature selectors. Also, the following transformers are considered as feature selectors:

  • FactorAnalysis
  • FastICA
  • IncrementalPCA
  • KernelPCA
  • LatentDirichletAllocation
  • MiniBatchDictionaryLearning
  • MiniBatchSparsePCA
  • NMF
  • PCA
  • SparsePCA
  • TruncatedSVD
  • VarianceThreshold
  • LocallyLinearEmbedding
  • Isomap
  • MDS
  • SpectralEmbedding
  • TSNE
  • sksurrogate.SensAprx
  • skrebate.ReliefF
  • skrebate.SURF
  • skrebate.SURFstar
  • skrebate.MultiSURF
  • skrebate.MultiSURFstar
  • skrebate.TuRF

imblearn pipelines

If an imblearn sampler is included in the config dictionary, then imblearn.pipeline.Pipeline will be used instead of sklearn.pipeline.Pipeline which enables the Pipeline to use imblearn samples too.

Categorical Variables

In case there are fields in the data that need to be treated as categorical, one could provide a list of indices through cat_cols. Then, the data will be transformed via category_encoders.one_hot.OneHotEncoder before being passed to the pipelines.