import numpy as np
import pandas as pd
from sklearn.base import BaseEstimator, TransformerMixin

class CaseNormalizer(BaseEstimator, TransformerMixin):
    def fit(self, X, y=None):
        return self

    def transform(self, X):
        return pd.Series(X).apply(lambda x: x.lower()).values

case_normalizer = CaseNormalizer()

X = np.array(['Implementing', 'a', 'Custom', 'Transformer', 'from', 'SCIKIT-LEARN'])
print(case_normalizer.transform(X))
