template_transformer.py

class TemplateTransformer(BaseEstimator, TransformerMixin):
    """ An example transformer that returns the element-wise square root."""
    def __init__(self, demo_param='demo'):
        self.demo_param = demo_param

    def fit(self, X, y=None):
        """A reference implementation of a fitting function for a transformer."""
        X = check_array(X, accept_sparse=True)

        self.n_features_ = X.shape[1]

        # Return the transformer
        return self

    def transform(self, X):
        """ A reference implementation of a transform function."""
        # Check is fit had been called
        check_is_fitted(self, 'n_features_')

        # Input validation
        X = check_array(X, accept_sparse=True)
        
        # Return transformed input
        return np.sqrt(X)