_label.py

# Authors: Alexandre Gramfort <alexandre.gramfort@inria.fr>
#          Mathieu Blondel <mathieu@mblondel.org>
#          Olivier Grisel <olivier.grisel@ensta.org>
#          Andreas Mueller <amueller@ais.uni-bonn.de>
#          Joel Nothman <joel.nothman@gmail.com>
#          Hamzeh Alsalhi <ha258@cornell.edu>
# License: BSD 3 clause

from collections import defaultdict
from numbers import Integral
import itertools
import array
import warnings

import numpy as np
import scipy.sparse as sp

from ..base import BaseEstimator, TransformerMixin

from ..utils.sparsefuncs import min_max_axis
from ..utils import column_or_1d
from ..utils.validation import _num_samples, check_array, check_is_fitted
from ..utils.multiclass import unique_labels
from ..utils.multiclass import type_of_target
from ..utils._encode import _encode, _unique


__all__ = [
    "label_binarize",
    "LabelBinarizer",
    "LabelEncoder",
    "MultiLabelBinarizer",
]


class LabelEncoder(TransformerMixin, BaseEstimator):
    """Encode target labels with value between 0 and n_classes-1.

    This transformer should be used to encode target values, *i.e.* `y`, and
    not the input `X`.

    Read more in the :ref:`User Guide <preprocessing_targets>`.

    .. versionadded:: 0.12

    Attributes
    ----------
    classes_ : ndarray of shape (n_classes,)
        Holds the label for each class.

    See Also
    --------
    OrdinalEncoder : Encode categorical features using an ordinal encoding
        scheme.
    OneHotEncoder : Encode categorical features as a one-hot numeric array.

    Examples
    --------
    `LabelEncoder` can be used to normalize labels.

    >>> from sklearn import preprocessing
    >>> le = preprocessing.LabelEncoder()
    >>> le.fit([1, 2, 2, 6])
    LabelEncoder()
    >>> le.classes_
    array([1, 2, 6])
    >>> le.transform([1, 1, 2, 6])
    array([0, 0, 1, 2]...)
    >>> le.inverse_transform([0, 0, 1, 2])
    array([1, 1, 2, 6])

    It can also be used to transform non-numerical labels (as long as they are
    hashable and comparable) to numerical labels.

    >>> le = preprocessing.LabelEncoder()
    >>> le.fit(["paris", "paris", "tokyo", "amsterdam"])
    LabelEncoder()
    >>> list(le.classes_)
    ['amsterdam', 'paris', 'tokyo']
    >>> le.transform(["tokyo", "tokyo", "paris"])
    array([2, 2, 1]...)
    >>> list(le.inverse_transform([2, 2, 1]))
    ['tokyo', 'tokyo', 'paris']
    """

    def fit(self, y):
        """Fit label encoder.

        Parameters
        ----------
        y : array-like of shape (n_samples,)
            Target values.

        Returns
        -------
        self : returns an instance of self.
            Fitted label encoder.
        """
        y = column_or_1d(y, warn=True)
        self.classes_ = _unique(y)
        return self

    def fit_transform(self, y):
        """Fit label encoder and return encoded labels.

        Parameters
        ----------
        y : array-like of shape (n_samples,)
            Target values.

        Returns
        -------
        y : array-like of shape (n_samples,)
            Encoded labels.
        """
        y = column_or_1d(y, warn=True)
        self.classes_, y = _unique(y, return_inverse=True)
        return y

    def transform(self, y):
        """Transform labels to normalized encoding.

        Parameters
        ----------
        y : array-like of shape (n_samples,)
            Target values.

        Returns
        -------
        y : array-like of shape (n_samples,)
            Labels as normalized encodings.
        """
        check_is_fitted(self)
        y = column_or_1d(y, dtype=self.classes_.dtype, warn=True)
        # transform of empty array is empty array
        if _num_samples(y) == 0:
            return np.array([])

        return _encode(y, uniques=self.classes_)

    def inverse_transform(self, y):
        """Transform labels back to original encoding.

        Parameters
        ----------
        y : ndarray of shape (n_samples,)
            Target values.

        Returns
        -------
        y : ndarray of shape (n_samples,)
            Original encoding.
        """
        check_is_fitted(self)
        y = column_or_1d(y, warn=True)
        # inverse transform of empty array is empty array
        if _num_samples(y) == 0:
            return np.array([])

        diff = np.setdiff1d(y, np.arange(len(self.classes_)))
        if len(diff):
            raise ValueError("y contains previously unseen labels: %s" % str(diff))
        y = np.asarray(y)
        return self.classes_[y]

    def _more_tags(self):
        return {"X_types": ["1dlabels"]}


class LabelBinarizer(TransformerMixin, BaseEstimator):
    """Binarize labels in a one-vs-all fashion.

    Several regression and binary classification algorithms are
    available in scikit-learn. A simple way to extend these algorithms
    to the multi-class classification case is to use the so-called
    one-vs-all scheme.

    At learning time, this simply consists in learning one regressor
    or binary classifier per class. In doing so, one needs to convert
    multi-class labels to binary labels (belong or does not belong
    to the class). LabelBinarizer makes this process easy with the
    transform method.

    At prediction time, one assigns the class for which the corresponding
    model gave the greatest confidence. LabelBinarizer makes this easy
    with the inverse_transform method.

    Read more in the :ref:`User Guide <preprocessing_targets>`.

    Parameters
    ----------
    neg_label : int, default=0
        Value with which negative labels must be encoded.

    pos_label : int, default=1
        Value with which positive labels must be encoded.

    sparse_output : bool, default=False
        True if the returned array from transform is desired to be in sparse
        CSR format.

    Attributes
    ----------
    classes_ : ndarray of shape (n_classes,)
        Holds the label for each class.

    y_type_ : str
        Represents the type of the target data as evaluated by
        utils.multiclass.type_of_target. Possible type are 'continuous',
        'continuous-multioutput', 'binary', 'multiclass',
        'multiclass-multioutput', 'multilabel-indicator', and 'unknown'.

    sparse_input_ : bool
        True if the input data to transform is given as a sparse matrix, False
        otherwise.

    See Also
    --------
    label_binarize : Function to perform the transform operation of
        LabelBinarizer with fixed classes.
    OneHotEncoder : Encode categorical features using a one-hot aka one-of-K
        scheme.

    Examples
    --------
    >>> from sklearn import preprocessing
    >>> lb = preprocessing.LabelBinarizer()
    >>> lb.fit([1, 2, 6, 4, 2])
    LabelBinarizer()
    >>> lb.classes_
    array([1, 2, 4, 6])
    >>> lb.transform([1, 6])
    array([[1, 0, 0, 0],
           [0, 0, 0, 1]])

    Binary targets transform to a column vector

    >>> lb = preprocessing.LabelBinarizer()
    >>> lb.fit_transform(['yes', 'no', 'no', 'yes'])
    array([[1],
           [0],
           [0],
           [1]])

    Passing a 2D matrix for multilabel classification

    >>> import numpy as np
    >>> lb.fit(np.array([[0, 1, 1], [1, 0, 0]]))
    LabelBinarizer()
    >>> lb.classes_
    array([0, 1, 2])
    >>> lb.transform([0, 1, 2, 1])
    array([[1, 0, 0],
           [0, 1, 0],
           [0, 0, 1],
           [0, 1, 0]])
    """

    _parameter_constraints: dict = {
        "neg_label": [Integral],
        "pos_label": [Integral],
        "sparse_output": ["boolean"],
    }

    def __init__(self, *, neg_label=0, pos_label=1, sparse_output=False):

        self.neg_label = neg_label
        self.pos_label = pos_label
        self.sparse_output = sparse_output

    def fit(self, y):
        """Fit label binarizer.

        Parameters
        ----------
        y : ndarray of shape (n_samples,) or (n_samples, n_classes)
            Target values. The 2-d matrix should only contain 0 and 1,
            represents multilabel classification.

        Returns
        -------
        self : object
            Returns the instance itself.
        """

        self._validate_params()

        if self.neg_label >= self.pos_label:
            raise ValueError(
                f"neg_label={self.neg_label} must be strictly less than "
                f"pos_label={self.pos_label}."
            )

        if self.sparse_output and (self.pos_label == 0 or self.neg_label != 0):
            raise ValueError(
                "Sparse binarization is only supported with non "
                "zero pos_label and zero neg_label, got "
                f"pos_label={self.pos_label} and neg_label={self.neg_label}"
            )

        self.y_type_ = type_of_target(y, input_name="y")

        if "multioutput" in self.y_type_:
            raise ValueError(
                "Multioutput target data is not supported with label binarization"
            )
        if _num_samples(y) == 0:
            raise ValueError("y has 0 samples: %r" % y)

        self.sparse_input_ = sp.issparse(y)
        self.classes_ = unique_labels(y)
        return self

    def fit_transform(self, y):
        """Fit label binarizer/transform multi-class labels to binary labels.

        The output of transform is sometimes referred to as
        the 1-of-K coding scheme.

        Parameters
        ----------
        y : {ndarray, sparse matrix} of shape (n_samples,) or \
                (n_samples, n_classes)
            Target values. The 2-d matrix should only contain 0 and 1,
            represents multilabel classification. Sparse matrix can be
            CSR, CSC, COO, DOK, or LIL.

        Returns
        -------
        Y : {ndarray, sparse matrix} of shape (n_samples, n_classes)
            Shape will be (n_samples, 1) for binary problems. Sparse matrix
            will be of CSR format.
        """
        return self.fit(y).transform(y)

    def transform(self, y):
        """Transform multi-class labels to binary labels.

        The output of transform is sometimes referred to by some authors as
        the 1-of-K coding scheme.

        Parameters
        ----------
        y : {array, sparse matrix} of shape (n_samples,) or \
                (n_samples, n_classes)
            Target values. The 2-d matrix should only contain 0 and 1,
            represents multilabel classification. Sparse matrix can be
            CSR, CSC, COO, DOK, or LIL.

        Returns
        -------
        Y : {ndarray, sparse matrix} of shape (n_samples, n_classes)
            Shape will be (n_samples, 1) for binary problems. Sparse matrix
            will be of CSR format.
        """
        check_is_fitted(self)

        y_is_multilabel = type_of_target(y).startswith("multilabel")
        if y_is_multilabel and not self.y_type_.startswith("multilabel"):
            raise ValueError("The object was not fitted with multilabel input.")

        return label_binarize(
            y,
            classes=self.classes_,
            pos_label=self.pos_label,
            neg_label=self.neg_label,
            sparse_output=self.sparse_output,
        )

    def inverse_transform(self, Y, threshold=None):
        """Transform binary labels back to multi-class labels.

        Parameters
        ----------
        Y : {ndarray, sparse matrix} of shape (n_samples, n_classes)
            Target values. All sparse matrices are converted to CSR before
            inverse transformation.

        threshold : float, default=None
            Threshold used in the binary and multi-label cases.

            Use 0 when ``Y`` contains the output of decision_function
            (classifier).
            Use 0.5 when ``Y`` contains the output of predict_proba.

            If None, the threshold is assumed to be half way between
            neg_label and pos_label.

        Returns
        -------
        y : {ndarray, sparse matrix} of shape (n_samples,)
            Target values. Sparse matrix will be of CSR format.

        Notes
        -----
        In the case when the binary labels are fractional
        (probabilistic), inverse_transform chooses the class with the
        greatest value. Typically, this allows to use the output of a
        linear model's decision_function method directly as the input
        of inverse_transform.
        """
        check_is_fitted(self)

        if threshold is None:
            threshold = (self.pos_label + self.neg_label) / 2.0

        if self.y_type_ == "multiclass":
            y_inv = _inverse_binarize_multiclass(Y, self.classes_)
        else:
            y_inv = _inverse_binarize_thresholding(
                Y, self.y_type_, self.classes_, threshold
            )

        if self.sparse_input_:
            y_inv = sp.csr_matrix(y_inv)
        elif sp.issparse(y_inv):
            y_inv = y_inv.toarray()

        return y_inv

    def _more_tags(self):
        return {"X_types": ["1dlabels"]}


def label_binarize(y, *, classes, neg_label=0, pos_label=1, sparse_output=False):
    """Binarize labels in a one-vs-all fashion.

    Several regression and binary classification algorithms are
    available in scikit-learn. A simple way to extend these algorithms
    to the multi-class classification case is to use the so-called
    one-vs-all scheme.

    This function makes it possible to compute this transformation for a
    fixed set of class labels known ahead of time.

    Parameters
    ----------
    y : array-like
        Sequence of integer labels or multilabel data to encode.

    classes : array-like of shape (n_classes,)
        Uniquely holds the label for each class.

    neg_label : int, default=0
        Value with which negative labels must be encoded.

    pos_label : int, default=1
        Value with which positive labels must be encoded.

    sparse_output : bool, default=False,
        Set to true if output binary array is desired in CSR sparse format.

    Returns
    -------
    Y : {ndarray, sparse matrix} of shape (n_samples, n_classes)
        Shape will be (n_samples, 1) for binary problems. Sparse matrix will
        be of CSR format.

    See Also
    --------
    LabelBinarizer : Class used to wrap the functionality of label_binarize and
        allow for fitting to classes independently of the transform operation.

    Examples
    --------
    >>> from sklearn.preprocessing import label_binarize
    >>> label_binarize([1, 6], classes=[1, 2, 4, 6])
    array([[1, 0, 0, 0],
           [0, 0, 0, 1]])

    The class ordering is preserved:

    >>> label_binarize([1, 6], classes=[1, 6, 4, 2])
    array([[1, 0, 0, 0],
           [0, 1, 0, 0]])

    Binary targets transform to a column vector

    >>> label_binarize(['yes', 'no', 'no', 'yes'], classes=['no', 'yes'])
    array([[1],
           [0],
           [0],
           [1]])
    """
    if not isinstance(y, list):
        # XXX Workaround that will be removed when list of list format is
        # dropped
        y = check_array(
            y, input_name="y", accept_sparse="csr", ensure_2d=False, dtype=None
        )
    else:
        if _num_samples(y) == 0:
            raise ValueError("y has 0 samples: %r" % y)
    if neg_label >= pos_label:
        raise ValueError(
            "neg_label={0} must be strictly less than pos_label={1}.".format(
                neg_label, pos_label
            )
        )

    if sparse_output and (pos_label == 0 or neg_label != 0):
        raise ValueError(
            "Sparse binarization is only supported with non "
            "zero pos_label and zero neg_label, got "
            "pos_label={0} and neg_label={1}"
            "".format(pos_label, neg_label)
        )

    # To account for pos_label == 0 in the dense case
    pos_switch = pos_label == 0
    if pos_switch:
        pos_label = -neg_label

    y_type = type_of_target(y)
    if "multioutput" in y_type:
        raise ValueError(
            "Multioutput target data is not supported with label binarization"
        )
    if y_type == "unknown":
        raise ValueError("The type of target data is not known")

    n_samples = y.shape[0] if sp.issparse(y) else len(y)
    n_classes = len(classes)
    classes = np.asarray(classes)

    if y_type == "binary":
        if n_classes == 1:
            if sparse_output:
                return sp.csr_matrix((n_samples, 1), dtype=int)
            else:
                Y = np.zeros((len(y), 1), dtype=int)
                Y += neg_label
                return Y
        elif len(classes) >= 3:
            y_type = "multiclass"

    sorted_class = np.sort(classes)
    if y_type == "multilabel-indicator":
        y_n_classes = y.shape[1] if hasattr(y, "shape") else len(y[0])
        if classes.size != y_n_classes:
            raise ValueError(
                "classes {0} mismatch with the labels {1} found in the data".format(
                    classes, unique_labels(y)
                )
            )

    if y_type in ("binary", "multiclass"):
        y = column_or_1d(y)

        # pick out the known labels from y
        y_in_classes = np.in1d(y, classes)
        y_seen = y[y_in_classes]
        indices = np.searchsorted(sorted_class, y_seen)
        indptr = np.hstack((0, np.cumsum(y_in_classes)))

        data = np.empty_like(indices)
        data.fill(pos_label)
        Y = sp.csr_matrix((data, indices, indptr), shape=(n_samples, n_classes))
    elif y_type == "multilabel-indicator":
        Y = sp.csr_matrix(y)
        if pos_label != 1:
            data = np.empty_like(Y.data)
            data.fill(pos_label)
            Y.data = data
    else:
        raise ValueError(
            "%s target data is not supported with label binarization" % y_type
        )

    if not sparse_output:
        Y = Y.toarray()
        Y = Y.astype(int, copy=False)

        if neg_label != 0:
            Y[Y == 0] = neg_label

        if pos_switch:
            Y[Y == pos_label] = 0
    else:
        Y.data = Y.data.astype(int, copy=False)

    # preserve label ordering
    if np.any(classes != sorted_class):
        indices = np.searchsorted(sorted_class, classes)
        Y = Y[:, indices]

    if y_type == "binary":
        if sparse_output:
            Y = Y.getcol(-1)
        else:
            Y = Y[:, -1].reshape((-1, 1))

    return Y


def _inverse_binarize_multiclass(y, classes):
    """Inverse label binarization transformation for multiclass.

    Multiclass uses the maximal score instead of a threshold.
    """
    classes = np.asarray(classes)

    if sp.issparse(y):
        # Find the argmax for each row in y where y is a CSR matrix

        y = y.tocsr()
        n_samples, n_outputs = y.shape
        outputs = np.arange(n_outputs)
        row_max = min_max_axis(y, 1)[1]
        row_nnz = np.diff(y.indptr)

        y_data_repeated_max = np.repeat(row_max, row_nnz)
        # picks out all indices obtaining the maximum per row
        y_i_all_argmax = np.flatnonzero(y_data_repeated_max == y.data)

        # For corner case where last row has a max of 0
        if row_max[-1] == 0:
            y_i_all_argmax = np.append(y_i_all_argmax, [len(y.data)])

        # Gets the index of the first argmax in each row from y_i_all_argmax
        index_first_argmax = np.searchsorted(y_i_all_argmax, y.indptr[:-1])
        # first argmax of each row
        y_ind_ext = np.append(y.indices, [0])
        y_i_argmax = y_ind_ext[y_i_all_argmax[index_first_argmax]]
        # Handle rows of all 0
        y_i_argmax[np.where(row_nnz == 0)[0]] = 0

        # Handles rows with max of 0 that contain negative numbers
        samples = np.arange(n_samples)[(row_nnz > 0) & (row_max.ravel() == 0)]
        for i in samples:
            ind = y.indices[y.indptr[i] : y.indptr[i + 1]]
            y_i_argmax[i] = classes[np.setdiff1d(outputs, ind)][0]

        return classes[y_i_argmax]
    else:
        return classes.take(y.argmax(axis=1), mode="clip")


def _inverse_binarize_thresholding(y, output_type, classes, threshold):
    """Inverse label binarization transformation using thresholding."""

    if output_type == "binary" and y.ndim == 2 and y.shape[1] > 2:
        raise ValueError("output_type='binary', but y.shape = {0}".format(y.shape))

    if output_type != "binary" and y.shape[1] != len(classes):
        raise ValueError(
            "The number of class is not equal to the number of dimension of y."
        )

    classes = np.asarray(classes)

    # Perform thresholding
    if sp.issparse(y):
        if threshold > 0:
            if y.format not in ("csr", "csc"):
                y = y.tocsr()
            y.data = np.array(y.data > threshold, dtype=int)
            y.eliminate_zeros()
        else:
            y = np.array(y.toarray() > threshold, dtype=int)
    else:
        y = np.array(y > threshold, dtype=int)

    # Inverse transform data
    if output_type == "binary":
        if sp.issparse(y):
            y = y.toarray()
        if y.ndim == 2 and y.shape[1] == 2:
            return classes[y[:, 1]]
        else:
            if len(classes) == 1:
                return np.repeat(classes[0], len(y))
            else:
                return classes[y.ravel()]

    elif output_type == "multilabel-indicator":
        return y

    else:
        raise ValueError("{0} format is not supported".format(output_type))


class MultiLabelBinarizer(TransformerMixin, BaseEstimator):
    """Transform between iterable of iterables and a multilabel format.

    Although a list of sets or tuples is a very intuitive format for multilabel
    data, it is unwieldy to process. This transformer converts between this
    intuitive format and the supported multilabel format: a (samples x classes)
    binary matrix indicating the presence of a class label.

    Parameters
    ----------
    classes : array-like of shape (n_classes,), default=None
        Indicates an ordering for the class labels.
        All entries should be unique (cannot contain duplicate classes).

    sparse_output : bool, default=False
        Set to True if output binary array is desired in CSR sparse format.

    Attributes
    ----------
    classes_ : ndarray of shape (n_classes,)
        A copy of the `classes` parameter when provided.
        Otherwise it corresponds to the sorted set of classes found
        when fitting.

    See Also
    --------
    OneHotEncoder : Encode categorical features using a one-hot aka one-of-K
        scheme.

    Examples
    --------
    >>> from sklearn.preprocessing import MultiLabelBinarizer
    >>> mlb = MultiLabelBinarizer()
    >>> mlb.fit_transform([(1, 2), (3,)])
    array([[1, 1, 0],
           [0, 0, 1]])
    >>> mlb.classes_
    array([1, 2, 3])

    >>> mlb.fit_transform([{'sci-fi', 'thriller'}, {'comedy'}])
    array([[0, 1, 1],
           [1, 0, 0]])
    >>> list(mlb.classes_)
    ['comedy', 'sci-fi', 'thriller']

    A common mistake is to pass in a list, which leads to the following issue:

    >>> mlb = MultiLabelBinarizer()
    >>> mlb.fit(['sci-fi', 'thriller', 'comedy'])
    MultiLabelBinarizer()
    >>> mlb.classes_
    array(['-', 'c', 'd', 'e', 'f', 'h', 'i', 'l', 'm', 'o', 'r', 's', 't',
        'y'], dtype=object)

    To correct this, the list of labels should be passed in as:

    >>> mlb = MultiLabelBinarizer()
    >>> mlb.fit([['sci-fi', 'thriller', 'comedy']])
    MultiLabelBinarizer()
    >>> mlb.classes_
    array(['comedy', 'sci-fi', 'thriller'], dtype=object)
    """

    _parameter_constraints: dict = {
        "classes": ["array-like", None],
        "sparse_output": ["boolean"],
    }

    def __init__(self, *, classes=None, sparse_output=False):
        self.classes = classes
        self.sparse_output = sparse_output

    def fit(self, y):
        """Fit the label sets binarizer, storing :term:`classes_`.

        Parameters
        ----------
        y : iterable of iterables
            A set of labels (any orderable and hashable object) for each
            sample. If the `classes` parameter is set, `y` will not be
            iterated.

        Returns
        -------
        self : object
            Fitted estimator.
        """
        self._validate_params()
        self._cached_dict = None

        if self.classes is None:
            classes = sorted(set(itertools.chain.from_iterable(y)))
        elif len(set(self.classes)) < len(self.classes):
            raise ValueError(
                "The classes argument contains duplicate "
                "classes. Remove these duplicates before passing "
                "them to MultiLabelBinarizer."
            )
        else:
            classes = self.classes
        dtype = int if all(isinstance(c, int) for c in classes) else object
        self.classes_ = np.empty(len(classes), dtype=dtype)
        self.classes_[:] = classes
        return self

    def fit_transform(self, y):
        """Fit the label sets binarizer and transform the given label sets.

        Parameters
        ----------
        y : iterable of iterables
            A set of labels (any orderable and hashable object) for each
            sample. If the `classes` parameter is set, `y` will not be
            iterated.

        Returns
        -------
        y_indicator : {ndarray, sparse matrix} of shape (n_samples, n_classes)
            A matrix such that `y_indicator[i, j] = 1` iff `classes_[j]`
            is in `y[i]`, and 0 otherwise. Sparse matrix will be of CSR
            format.
        """
        if self.classes is not None:
            return self.fit(y).transform(y)

        self._validate_params()
        self._cached_dict = None

        # Automatically increment on new class
        class_mapping = defaultdict(int)
        class_mapping.default_factory = class_mapping.__len__
        yt = self._transform(y, class_mapping)

        # sort classes and reorder columns
        tmp = sorted(class_mapping, key=class_mapping.get)

        # (make safe for tuples)
        dtype = int if all(isinstance(c, int) for c in tmp) else object
        class_mapping = np.empty(len(tmp), dtype=dtype)
        class_mapping[:] = tmp
        self.classes_, inverse = np.unique(class_mapping, return_inverse=True)
        # ensure yt.indices keeps its current dtype
        yt.indices = np.array(inverse[yt.indices], dtype=yt.indices.dtype, copy=False)

        if not self.sparse_output:
            yt = yt.toarray()

        return yt

    def transform(self, y):
        """Transform the given label sets.

        Parameters
        ----------
        y : iterable of iterables
            A set of labels (any orderable and hashable object) for each
            sample. If the `classes` parameter is set, `y` will not be
            iterated.

        Returns
        -------
        y_indicator : array or CSR matrix, shape (n_samples, n_classes)
            A matrix such that `y_indicator[i, j] = 1` iff `classes_[j]` is in
            `y[i]`, and 0 otherwise.
        """
        check_is_fitted(self)

        class_to_index = self._build_cache()
        yt = self._transform(y, class_to_index)

        if not self.sparse_output:
            yt = yt.toarray()

        return yt

    def _build_cache(self):
        if self._cached_dict is None:
            self._cached_dict = dict(zip(self.classes_, range(len(self.classes_))))

        return self._cached_dict

    def _transform(self, y, class_mapping):
        """Transforms the label sets with a given mapping.

        Parameters
        ----------
        y : iterable of iterables
            A set of labels (any orderable and hashable object) for each
            sample. If the `classes` parameter is set, `y` will not be
            iterated.

        class_mapping : Mapping
            Maps from label to column index in label indicator matrix.

        Returns
        -------
        y_indicator : sparse matrix of shape (n_samples, n_classes)
            Label indicator matrix. Will be of CSR format.
        """
        indices = array.array("i")
        indptr = array.array("i", [0])
        unknown = set()
        for labels in y:
            index = set()
            for label in labels:
                try:
                    index.add(class_mapping[label])
                except KeyError:
                    unknown.add(label)
            indices.extend(index)
            indptr.append(len(indices))
        if unknown:
            warnings.warn(
                "unknown class(es) {0} will be ignored".format(sorted(unknown, key=str))
            )
        data = np.ones(len(indices), dtype=int)

        return sp.csr_matrix(
            (data, indices, indptr), shape=(len(indptr) - 1, len(class_mapping))
        )

    def inverse_transform(self, yt):
        """Transform the given indicator matrix into label sets.

        Parameters
        ----------
        yt : {ndarray, sparse matrix} of shape (n_samples, n_classes)
            A matrix containing only 1s ands 0s.

        Returns
        -------
        y : list of tuples
            The set of labels for each sample such that `y[i]` consists of
            `classes_[j]` for each `yt[i, j] == 1`.
        """
        check_is_fitted(self)

        if yt.shape[1] != len(self.classes_):
            raise ValueError(
                "Expected indicator for {0} classes, but got {1}".format(
                    len(self.classes_), yt.shape[1]
                )
            )

        if sp.issparse(yt):
            yt = yt.tocsr()
            if len(yt.data) != 0 and len(np.setdiff1d(yt.data, [0, 1])) > 0:
                raise ValueError("Expected only 0s and 1s in label indicator.")
            return [
                tuple(self.classes_.take(yt.indices[start:end]))
                for start, end in zip(yt.indptr[:-1], yt.indptr[1:])
            ]
        else:
            unexpected = np.setdiff1d(yt, [0, 1])
            if len(unexpected) > 0:
                raise ValueError(
                    "Expected only 0s and 1s in label indicator. Also got {0}".format(
                        unexpected
                    )
                )
            return [tuple(self.classes_.compress(indicators)) for indicators in yt]

    def _more_tags(self):
        return {"X_types": ["2dlabels"]}