(and thus a 2-dimensional array-like for a set of samples). In other

words, it (lossily) maps a non-rectangular data representation into

:term:`rectangular` data.

Feature extractors must implement at least:

* :term:`fit`

:term:`set_params`. A parameter ``deep`` can be used, when set to

False to only return those parameters not including ``__``, i.e. not

due to indirection via contained estimators.

Most estimators adopt the definition from :class:`base.BaseEstimator`,

which simply adopts the parameters defined for ``__init__``.

:class:`pipeline.Pipeline`, among others, reimplements ``get_params``

of the values in the classifier's :term:`classes_` attribute.

clusterer

``n_clusters - 1`` if the corresponding sample is clustered,

and -1 if the sample is not clustered, as in

:func:`cluster.dbscan`.