This is not suitable for encoding classification labels, but for features.

See also #9614

agreed that this is for features and should be in preprocessing/.
#9614 is very related, but it creates three arrays for every array you send it. This is completely different than LabelEncoder.

would it make sense to try to steer that PR to a simpler API?

#9614 is very related, but it creates three arrays for every array you send it.

Can you elaborate on that? I don't think I understand.

Sure! From line 2935 of this file it appears that when CountFeaturizer.fit_transform is sent an array of shape (N, 1) it returns an array of shape (N, 3).

Another difference is that CountFeaturizer takes an inclusion argument: 'all', 'each', list, or numpy.ndarray, whereas in my experience most preprocessors preprocess all the data they are sent.

hm I think the interface of the CountVectorizer is a bit odd right now and I'd change some of that.
My expectation would be that in a multi-class setting you have n_classes many columns. How would you reduce it to a single column?

Something like

class FrequencyEncoder(BaseEstimator, TransformerMixin):
    """If a value appears n times in a column sent to fit, it is encoded as n."""

    def fit(self, X):
        self._counts = [Counter(X[:, i] for i in range(X.shape[1]))]
        return self

    def transform(self, X):
		# try to broadcast more
        return np.array([[self._counts[i][elem]
						  for elem in X[:, i]]
                          for i in range(X.shape[1])])

CountVectorizer changes could also be interesting. I am trying to transform features, not labels, though so I don't think whether the setting is multi-class matters?

My theory is that for a tree model, this would be like using OrdinalEncoder to transform categoricals, but provide more information than just the order of appearance.

if our data is [red, red, red, red, yellow, red, green, green, green], replacing red with 1 and yellow with 2 and green with 3 tells the model less than replacing them with 4, 1, 3. CountFeaturizer provides even more info at the cost of more features.

I can work on providing empirical proof, if that would help, but also happy to give up and look for other stuff to do!