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Estimator creating _more_tags and inheriting from BaseEstimator will not warn about old tag infrastructure #30257

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glemaitre opened this issue Nov 9, 2024 · 4 comments · Fixed by #30327
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Estimator creating _more_tags and inheriting from BaseEstimator will not warn about old tag infrastructure #30257

glemaitre opened this issue Nov 9, 2024 · 4 comments · Fixed by #30327
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@glemaitre
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While making the code of skrub compatible with scikit-learn 1.6, I found that the following is really surprising:

# %%
import numpy as np
from sklearn.base import BaseEstimator, RegressorMixin

class MyRegressor(RegressorMixin, BaseEstimator):
    def __init__(self, seed=None):
        self.seed = seed

    def fit(self, X, y):
        self.rng_ = np.random.default_rng(self.seed)
        return self

    def predict(self, X):
        return self.rng_.normal(size=X.shape[0])

    def _more_tags(self):
        return {
            "multioutput": True
        }


# %%
from sklearn.datasets import make_regression

X, y = make_regression(n_samples=10, n_features=5, random_state=42)
regressor = MyRegressor(seed=42).fit(X, y)
regressor.predict(X)

# %%
from sklearn.utils import get_tags

tags = get_tags(regressor)  # does not warn because we inherit from BaseEstimator
tags.target_tags.multi_output  # does not use anymore the _more_tags and thus is wrong

In the code above, because we inherit from BaseEstimator and RegressorMixin, we have the default tags set with the methods __sklearn_tags__.

However, the previous code that we had was using _more_tags.

Currently, get_tags will not warn that something is going wrong because we will fallback on the default tags from the base class and mixins.

I think that we should:

  • use the values defined in _more_tags and warn for the future change
  • in the future we should error if we have both _more_tags and __sklearn_tags__ to be sure that people stop using _more_tags
@glemaitre glemaitre added this to the 1.6 milestone Nov 9, 2024
@glemaitre glemaitre mentioned this issue Nov 11, 2024
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@thomasjpfan
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use the values defined in _more_tags and warn for the future change

There are two paths to continue using _more_tags:

  1. Bring back all the _get_tags + _more_tags logic from before. Bringing back _get_tags will introduce some complexity when both _get_tags and __sklearn_tags__ are defined.
  2. Only support _more_tags. If _more_tags exist, then have __sklearn_tags__ create a Tag object that uses the configuration from _more_tags. (This means we'll need to translate from the old dict format to the new dataclasses.)

I'm in-favor of 2.

@ogrisel
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ogrisel commented Nov 22, 2024

I am also in favor 2.

@glemaitre
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Oh I forgot about this issue.

@glemaitre
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glemaitre commented Nov 22, 2024 •
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So this issue really happen for third-party developer: #30324

There, I could witness that #30327 is solving the issue.

#30327 is reintroducing _get_tags because we will still break code of people. So there is definitely a complexity because you need to convert __sklearn_tags__ that do inheritance into a _more_tags that just add tag for the class itself. But I think that I got it right with the different test. It really provide a fully backward compatible solution.

And I'm not sure that option would be enough. Translating _more_tags to a Tags means that you will create some default values. Since __sklearn_tags__ use the inheritance, having the default tags will wrongly override the class of a child class I think.

Edit: actually @thomasjpfan, I was thinking the walk in the MRO reversed :).

@glemaitre glemaitre mentioned this issue Nov 22, 2024
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MAINT conversion old->new/new->old tags (bis) glemaitre/scikit-learn
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@ogrisel @thomasjpfan @glemaitre