SLEP013: n_features_out_ attribute

Author:Adrin Jalali
Status:Under Review
Type:Standards Track


This SLEP proposes the introduction of a public n_features_out_ attribute for most transformers (where relevant).


Knowing the number of features that a transformer outputs is useful for inspection purposes. This is in conjunction with *SLEP010: ``n_features_in_``*.


The proposed solution is for the n_features_out_ attribute to be set once a call to fit is done. In many cases the value of n_features_out_ is the same as some other attribute stored in the transformer, e.g. n_components_, and in these cases a Mixin such as a ComponentsMixin can delegate n_features_out_ to those attributes.


A test to the common tests is added to ensure the presence of the attribute or property after calling fit.


The main consideration is that the addition of the common test means that existing estimators in downstream libraries will not pass our test suite, unless the estimators also have the n_features_out_ attribute.

The newly introduced checks will only raise a warning instead of an exception for the next 2 releases, so this will give more time for downstream packages to adjust.

There are other minor considerations:

  • In some meta-estimators, this is delegated to the sub-estimator(s). The n_features_out_ attribute of the meta-estimator is thus explicitly set to that of the sub-estimator, either via a @property, or directly in fit().
  • Some transformers such as FunctionTransformer may not know the number of output features since arbitrary arrays can be passed to transform. In such cases n_features_out_ is set to None.

References and Footnotes

[1]_Open Publication License: