future_encoders.py

# Authors: Andreas Mueller <amueller@ais.uni-bonn.de>
#          Joris Van den Bossche <jorisvandenbossche@gmail.com>
# License: BSD 3 clause

from __future__ import division

import numbers
import warnings

import numpy as np
from scipy import sparse

from sklearn.base import BaseEstimator, TransformerMixin
from sklearn.externals import six
from sklearn.utils import check_array
from sklearn.utils.validation import check_is_fitted, FLOAT_DTYPES
from sklearn.preprocessing.label import LabelEncoder


BOUNDS_THRESHOLD = 1e-7


zip = six.moves.zip
map = six.moves.map
range = six.moves.range

__all__ = [
    'OneHotEncoder',
    'OrdinalEncoder'
]


def _argmax(arr_or_spmatrix, axis=None):
    return arr_or_spmatrix.argmax(axis=axis)


def _handle_zeros_in_scale(scale, copy=True):
    ''' Makes sure that whenever scale is zero, we handle it correctly.

    This happens in most scalers when we have constant features.'''

    # if we are fitting on 1D arrays, scale might be a scalar
    if np.isscalar(scale):
        if scale == .0:
            scale = 1.
        return scale
    elif isinstance(scale, np.ndarray):
        if copy:
            # New array to avoid side-effects
            scale = scale.copy()
        scale[scale == 0.0] = 1.0
        return scale


def _transform_selected(X, transform, selected="all", copy=True):
    """Apply a transform function to portion of selected features

    Parameters
    ----------
    X : {array-like, sparse matrix}, shape [n_samples, n_features]
        Dense array or sparse matrix.

    transform : callable
        A callable transform(X) -> X_transformed

    copy : boolean, optional
        Copy X even if it could be avoided.

    selected: "all" or array of indices or mask
        Specify which features to apply the transform to.

    Returns
    -------
    X : array or sparse matrix, shape=(n_samples, n_features_new)
    """
    X = check_array(X, accept_sparse='csc', copy=copy, dtype=FLOAT_DTYPES)

    if isinstance(selected, six.string_types) and selected == "all":
        return transform(X)

    if len(selected) == 0:
        return X

    n_features = X.shape[1]
    ind = np.arange(n_features)
    sel = np.zeros(n_features, dtype=bool)
    sel[np.asarray(selected)] = True
    not_sel = np.logical_not(sel)
    n_selected = np.sum(sel)

    if n_selected == 0:
        # No features selected.
        return X
    elif n_selected == n_features:
        # All features selected.
        return transform(X)
    else:
        X_sel = transform(X[:, ind[sel]])
        X_not_sel = X[:, ind[not_sel]]

        if sparse.issparse(X_sel) or sparse.issparse(X_not_sel):
            return sparse.hstack((X_sel, X_not_sel))
        else:
            return np.hstack((X_sel, X_not_sel))


class _BaseEncoder(BaseEstimator, TransformerMixin):
    """
    Base class for encoders that includes the code to categorize and
    transform the input features.

    """

    def _fit(self, X, handle_unknown='error'):

        X_temp = check_array(X, dtype=None)
        if not hasattr(X, 'dtype') and np.issubdtype(X_temp.dtype, np.str_):
            X = check_array(X, dtype=np.object)
        else:
            X = X_temp

        n_samples, n_features = X.shape

        if self.categories != 'auto':
            for cats in self.categories:
                if not np.all(np.sort(cats) == np.array(cats)):
                    raise ValueError("Unsorted categories are not yet "
                                     "supported")
            if len(self.categories) != n_features:
                raise ValueError("Shape mismatch: if n_values is an array,"
                                 " it has to be of shape (n_features,).")

        self._label_encoders_ = [LabelEncoder() for _ in range(n_features)]

        for i in range(n_features):
            le = self._label_encoders_[i]
            Xi = X[:, i]
            if self.categories == 'auto':
                le.fit(Xi)
            else:
                if handle_unknown == 'error':
                    valid_mask = np.in1d(Xi, self.categories[i])
                    if not np.all(valid_mask):
                        diff = np.unique(Xi[~valid_mask])
                        msg = ("Found unknown categories {0} in column {1}"
                               " during fit".format(diff, i))
                        raise ValueError(msg)
                le.classes_ = np.array(self.categories[i])

        self.categories_ = [le.classes_ for le in self._label_encoders_]

    def _transform(self, X, handle_unknown='error'):

        X_temp = check_array(X, dtype=None)
        if not hasattr(X, 'dtype') and np.issubdtype(X_temp.dtype, np.str_):
            X = check_array(X, dtype=np.object)
        else:
            X = X_temp

        _, n_features = X.shape
        X_int = np.zeros_like(X, dtype=np.int)
        X_mask = np.ones_like(X, dtype=np.bool)

        for i in range(n_features):
            Xi = X[:, i]
            valid_mask = np.in1d(Xi, self.categories_[i])

            if not np.all(valid_mask):
                if handle_unknown == 'error':
                    diff = np.unique(X[~valid_mask, i])
                    msg = ("Found unknown categories {0} in column {1}"
                           " during transform".format(diff, i))
                    raise ValueError(msg)
                else:
                    # Set the problematic rows to an acceptable value and
                    # continue `The rows are marked `X_mask` and will be
                    # removed later.
                    X_mask[:, i] = valid_mask
                    Xi = Xi.copy()
                    Xi[~valid_mask] = self.categories_[i][0]
            X_int[:, i] = self._label_encoders_[i].transform(Xi)

        return X_int, X_mask


WARNING_MSG = (
    "The handling of integer data will change in the future. Currently, the "
    "categories are determined based on the range [0, max(values)], while "
    "in the future they will be determined based on the unique values.\n"
    "If you want the future behaviour, you can specify \"categories='auto'\"."
)


class OneHotEncoder(_BaseEncoder):
    """Encode categorical integer features as a one-hot numeric array.

    The input to this transformer should be an array-like of integers or
    strings, denoting the values taken on by categorical (discrete) features.
    The features are encoded using a one-hot (aka 'one-of-K' or 'dummy')
    encoding scheme. This creates a binary column for each category and
    returns a sparse matrix or dense array.

    By default, the encoder derives the categories based on the unique values
    in each feature. Alternatively, you can also specify the `categories`
    manually.
    The OneHotEncoder previously assumed that the input features take on
    values in the range [0, max(values)). This behaviour is deprecated.

    This encoding is needed for feeding categorical data to many scikit-learn
    estimators, notably linear models and SVMs with the standard kernels.

    Note: a one-hot encoding of y labels should use a LabelBinarizer
    instead.

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

    Parameters
    ----------
    categories : 'auto' or a list of lists/arrays of values.
        Categories (unique values) per feature:

        - 'auto' : Determine categories automatically from the training data.
        - list : ``categories[i]`` holds the categories expected in the ith
          column. The passed categories must be sorted and should not mix
          strings and numeric values.

        The used categories can be found in the ``categories_`` attribute.

    sparse : boolean, default=True
        Will return sparse matrix if set True else will return an array.

    dtype : number type, default=np.float
        Desired dtype of output.

    handle_unknown : 'error' (default) or 'ignore'
        Whether to raise an error or ignore if a unknown categorical feature is
        present during transform (default is to raise). When this parameter
        is set to 'ignore' and an unknown category is encountered during
        transform, the resulting one-hot encoded columns for this feature
        will be all zeros. In the inverse transform, an unknown category
        will be denoted as None.

    n_values : 'auto', int or array of ints
        Number of values per feature.

        - 'auto' : determine value range from training data.
        - int : number of categorical values per feature.
                Each feature value should be in ``range(n_values)``
        - array : ``n_values[i]`` is the number of categorical values in
                  ``X[:, i]``. Each feature value should be
                  in ``range(n_values[i])``

        .. deprecated:: 0.20
            The `n_values` keyword is deprecated and will be removed in 0.22.
            Use `categories` instead.

    categorical_features : "all" or array of indices or mask
        Specify what features are treated as categorical.

        - 'all' (default): All features are treated as categorical.
        - array of indices: Array of categorical feature indices.
        - mask: Array of length n_features and with dtype=bool.

        Non-categorical features are always stacked to the right of the matrix.

        .. deprecated:: 0.20
            The `categorical_features` keyword is deprecated and will be
            removed in 0.22.

    Attributes
    ----------
    categories_ : list of arrays
        The categories of each feature determined during fitting
        (in order corresponding with output of ``transform``).

    active_features_ : array
        Indices for active features, meaning values that actually occur
        in the training set. Only available when n_values is ``'auto'``.

        .. deprecated:: 0.20

    feature_indices_ : array of shape (n_features,)
        Indices to feature ranges.
        Feature ``i`` in the original data is mapped to features
        from ``feature_indices_[i]`` to ``feature_indices_[i+1]``
        (and then potentially masked by `active_features_` afterwards)

        .. deprecated:: 0.20

    n_values_ : array of shape (n_features,)
        Maximum number of values per feature.

        .. deprecated:: 0.20

    Examples
    --------
    Given a dataset with two features, we let the encoder find the unique
    values per feature and transform the data to a binary one-hot encoding.

    >>> from sklearn.preprocessing import OneHotEncoder
    >>> enc = OneHotEncoder(handle_unknown='ignore')
    >>> X = [['Male', 1], ['Female', 3], ['Female', 2]]
    >>> enc.fit(X)
    ... # doctest: +ELLIPSIS
    OneHotEncoder(categories='auto', dtype=<... 'numpy.float64'>,
           handle_unknown='ignore', sparse=True)

    >>> enc.categories_
    [array(['Female', 'Male'], dtype=object), array([1, 2, 3], dtype=object)]
    >>> enc.transform([['Female', 1], ['Male', 4]]).toarray()
    array([[ 1.,  0.,  1.,  0.,  0.],
           [ 0.,  1.,  0.,  0.,  0.]])
    >>> enc.inverse_transform([[0, 1, 1, 0, 0], [0, 0, 0, 1, 0]])
    array([['Male', 1],
           [None, 2]], dtype=object)

    See also
    --------
    sklearn.preprocessing.OrdinalEncoder : performs an ordinal (integer)
      encoding of the categorical features.
    sklearn.feature_extraction.DictVectorizer : performs a one-hot encoding of
      dictionary items (also handles string-valued features).
    sklearn.feature_extraction.FeatureHasher : performs an approximate one-hot
      encoding of dictionary items or strings.
    sklearn.preprocessing.LabelBinarizer : binarizes labels in a one-vs-all
      fashion.
    sklearn.preprocessing.MultiLabelBinarizer : transforms between iterable of
      iterables and a multilabel format, e.g. a (samples x classes) binary
      matrix indicating the presence of a class label.
    """

    def __init__(self, n_values=None, categorical_features=None,
                 categories=None, sparse=True, dtype=np.float64,
                 handle_unknown='error'):
        self._categories = categories
        if categories is None:
            self.categories = 'auto'
        else:
            self.categories = categories
        self.sparse = sparse
        self.dtype = dtype
        self.handle_unknown = handle_unknown

        if n_values is not None:
            pass
            # warnings.warn("Deprecated", DeprecationWarning)
        else:
            n_values = "auto"
        self._deprecated_n_values = n_values

        if categorical_features is not None:
            pass
            # warnings.warn("Deprecated", DeprecationWarning)
        else:
            categorical_features = "all"
        self._deprecated_categorical_features = categorical_features

    # Deprecated keywords

    @property
    def n_values(self):
        warnings.warn("The 'n_values' parameter is deprecated.",
                      DeprecationWarning)
        return self._deprecated_n_values

    @n_values.setter
    def n_values(self, value):
        warnings.warn("The 'n_values' parameter is deprecated.",
                      DeprecationWarning)
        self._deprecated_n_values = value

    @property
    def categorical_features(self):
        warnings.warn("The 'categorical_features' parameter is deprecated.",
                      DeprecationWarning)
        return self._deprecated_categorical_features

    @categorical_features.setter
    def categorical_features(self, value):
        warnings.warn("The 'categorical_features' parameter is deprecated.",
                      DeprecationWarning)
        self._deprecated_categorical_features = value

    # Deprecated attributes

    @property
    def active_features_(self):
        check_is_fitted(self, 'categories_')
        warnings.warn("The 'active_features_' attribute is deprecated.",
                      DeprecationWarning)
        return self._active_features_

    @property
    def feature_indices_(self):
        check_is_fitted(self, 'categories_')
        warnings.warn("The 'feature_indices_' attribute is deprecated.",
                      DeprecationWarning)
        return self._feature_indices_

    @property
    def n_values_(self):
        check_is_fitted(self, 'categories_')
        warnings.warn("The 'n_values_' attribute is deprecated.",
                      DeprecationWarning)
        return self._n_values_

    def _handle_deprecations(self, X):

        user_set_categories = False

        if self._categories is not None:
            self._legacy_mode = False
            user_set_categories = True

        elif self._deprecated_n_values != 'auto':
            msg = (
                "Passing 'n_values' is deprecated and will be removed in a "
                "future release. You can use the 'categories' keyword instead."
                " 'n_values=n' corresponds to 'n_values=[range(n)]'.")
            warnings.warn(msg, DeprecationWarning)

            # we internally translate this to the correct categories
            # and don't use legacy mode
            X = check_array(X, dtype=np.int)

            if isinstance(self._deprecated_n_values, numbers.Integral):
                n_features = X.shape[1]
                self.categories = [
                    list(range(self._deprecated_n_values))
                    for _ in range(n_features)]
                n_values = np.empty(n_features, dtype=np.int)
                n_values.fill(self._deprecated_n_values)
            else:
                try:
                    n_values = np.asarray(self._deprecated_n_values, dtype=int)
                    self.categories = [list(range(i))
                                       for i in self._deprecated_n_values]
                except (ValueError, TypeError):
                    raise TypeError(
                        "Wrong type for parameter `n_values`. Expected 'auto',"
                        " int or array of ints, got %r".format(type(X)))

            self._n_values_ = n_values
            n_values = np.hstack([[0], n_values])
            indices = np.cumsum(n_values)
            self._feature_indices_ = indices

            self._legacy_mode = False

        else:  # n_values = 'auto'
            if self.handle_unknown == 'ignore':
                # no change in behaviour, no need to raise deprecation warning
                self._legacy_mode = False
            else:

                # check if we have integer or categorical input
                try:
                    X = check_array(X, dtype=np.int)
                except ValueError:
                    self._legacy_mode = False
                else:
                    warnings.warn(WARNING_MSG, DeprecationWarning)
                    self._legacy_mode = True

        if (not isinstance(self._deprecated_categorical_features,
                           six.string_types)
                or (isinstance(self._deprecated_categorical_features,
                               six.string_types)
                    and self._deprecated_categorical_features != 'all')):
            if user_set_categories:
                raise ValueError(
                    "The 'categorical_features' keyword is deprecated, and "
                    "cannot be used together with specifying 'categories'.")
            warnings.warn("The 'categorical_features' keyword is deprecated.",
                          DeprecationWarning)
            self._legacy_mode = True

    def fit(self, X, y=None):
        """Fit OneHotEncoder to X.

        Parameters
        ----------
        X : array-like, shape [n_samples, n_feature]
            The data to determine the categories of each feature.

        Returns
        -------
        self
        """
        if self.handle_unknown not in ['error', 'ignore']:
            template = ("handle_unknown should be either 'error' or "
                        "'ignore', got %s")
            raise ValueError(template % self.handle_unknown)

        self._handle_deprecations(X)

        if self._legacy_mode:
            # TODO not with _transform_selected ??
            self._legacy_fit_transform(X)
            return self
        else:
            self._fit(X, handle_unknown=self.handle_unknown)
            return self

    def _legacy_fit_transform(self, X):
        """Assumes X contains only categorical features."""
        self_n_values = self._deprecated_n_values
        dtype = getattr(X, 'dtype', None)
        X = check_array(X, dtype=np.int)
        if np.any(X < 0):
            raise ValueError("X needs to contain only non-negative integers.")
        n_samples, n_features = X.shape
        if (isinstance(self_n_values, six.string_types) and
                self_n_values == 'auto'):
            n_values = np.max(X, axis=0) + 1
        elif isinstance(self_n_values, numbers.Integral):
            if (np.max(X, axis=0) >= self_n_values).any():
                raise ValueError("Feature out of bounds for n_values=%d"
                                 % self_n_values)
            n_values = np.empty(n_features, dtype=np.int)
            n_values.fill(self_n_values)
        else:
            try:
                n_values = np.asarray(self_n_values, dtype=int)
            except (ValueError, TypeError):
                raise TypeError("Wrong type for parameter `n_values`. Expected"
                                " 'auto', int or array of ints, got %r"
                                % type(X))
            if n_values.ndim < 1 or n_values.shape[0] != X.shape[1]:
                raise ValueError("Shape mismatch: if n_values is an array,"
                                 " it has to be of shape (n_features,).")

        self._n_values_ = n_values
        self.categories_ = [np.arange(n_val - 1, dtype=dtype)
                            for n_val in n_values]
        n_values = np.hstack([[0], n_values])
        indices = np.cumsum(n_values)
        self._feature_indices_ = indices

        column_indices = (X + indices[:-1]).ravel()
        row_indices = np.repeat(np.arange(n_samples, dtype=np.int32),
                                n_features)
        data = np.ones(n_samples * n_features)
        out = sparse.coo_matrix((data, (row_indices, column_indices)),
                                shape=(n_samples, indices[-1]),
                                dtype=self.dtype).tocsr()

        if (isinstance(self_n_values, six.string_types) and
                self_n_values == 'auto'):
            mask = np.array(out.sum(axis=0)).ravel() != 0
            active_features = np.where(mask)[0]
            out = out[:, active_features]
            self._active_features_ = active_features

            self.categories_ = [
                np.unique(X[:, i]).astype(dtype) if dtype else np.unique(X[:, i])
                for i in range(n_features)]
            #import pdb; pdb.set_trace()

        return out if self.sparse else out.toarray()

    def fit_transform(self, X, y=None):
        """Fit OneHotEncoder to X, then transform X.

        Equivalent to self.fit(X).transform(X), but more convenient and more
        efficient. See fit for the parameters, transform for the return value.

        Parameters
        ----------
        X : array-like, shape [n_samples, n_feature]
            Input array of type int.
        """
        if self.handle_unknown not in ['error', 'ignore']:
            template = ("handle_unknown should be either 'error' or "
                        "'ignore', got %s")
            raise ValueError(template % self.handle_unknown)

        self._handle_deprecations(X)

        if self._legacy_mode:
            return _transform_selected(X, self._legacy_fit_transform,
                                       self._deprecated_categorical_features,
                                       copy=True)
        else:
            return self.fit(X).transform(X)

    def _legacy_transform(self, X):
        """Assumes X contains only categorical features."""
        self_n_values = self._deprecated_n_values
        X = check_array(X, dtype=np.int)
        if np.any(X < 0):
            raise ValueError("X needs to contain only non-negative integers.")
        n_samples, n_features = X.shape

        indices = self._feature_indices_
        if n_features != indices.shape[0] - 1:
            raise ValueError("X has different shape than during fitting."
                             " Expected %d, got %d."
                             % (indices.shape[0] - 1, n_features))

        # We use only those categorical features of X that are known using fit.
        # i.e lesser than n_values_ using mask.
        # This means, if self.handle_unknown is "ignore", the row_indices and
        # col_indices corresponding to the unknown categorical feature are
        # ignored.
        mask = (X < self._n_values_).ravel()
        if np.any(~mask):
            if self.handle_unknown not in ['error', 'ignore']:
                raise ValueError("handle_unknown should be either error or "
                                 "unknown got %s" % self.handle_unknown)
            if self.handle_unknown == 'error':
                raise ValueError("unknown categorical feature present %s "
                                 "during transform." % X.ravel()[~mask])

        column_indices = (X + indices[:-1]).ravel()[mask]
        row_indices = np.repeat(np.arange(n_samples, dtype=np.int32),
                                n_features)[mask]
        data = np.ones(np.sum(mask))
        out = sparse.coo_matrix((data, (row_indices, column_indices)),
                                shape=(n_samples, indices[-1]),
                                dtype=self.dtype).tocsr()
        if (isinstance(self_n_values, six.string_types) and
                self_n_values == 'auto'):
            out = out[:, self._active_features_]

        return out if self.sparse else out.toarray()

    def _transform_new(self, X):
        """New implementation assuming categorical input"""
        X_temp = check_array(X, dtype=None)
        if not hasattr(X, 'dtype') and np.issubdtype(X_temp.dtype, np.str_):
            X = check_array(X, dtype=np.object)
        else:
            X = X_temp

        n_samples, n_features = X.shape

        X_int, X_mask = self._transform(X, handle_unknown=self.handle_unknown)

        mask = X_mask.ravel()
        n_values = [cats.shape[0] for cats in self.categories_]
        n_values = np.array([0] + n_values)
        feature_indices = np.cumsum(n_values)

        indices = (X_int + feature_indices[:-1]).ravel()[mask]
        indptr = X_mask.sum(axis=1).cumsum()
        indptr = np.insert(indptr, 0, 0)
        data = np.ones(n_samples * n_features)[mask]

        out = sparse.csr_matrix((data, indices, indptr),
                                shape=(n_samples, feature_indices[-1]),
                                dtype=self.dtype)
        if not self.sparse:
            return out.toarray()
        else:
            return out

    def transform(self, X):
        """Transform X using one-hot encoding.

        Parameters
        ----------
        X : array-like, shape [n_samples, n_features]
            The data to encode.

        Returns
        -------
        X_out : sparse matrix if sparse=True else a 2-d array
            Transformed input.
        """
        if not self._legacy_mode:
            return self._transform_new(X)
        else:
            return _transform_selected(X, self._legacy_transform,
                                       self._deprecated_categorical_features,
                                       copy=True)

    def inverse_transform(self, X):
        """Convert back the data to the original representation.

        In case unknown categories are encountered (all zero's in the
        one-hot encoding), ``None`` is used to represent this category.

        Parameters
        ----------
        X : array-like or sparse matrix, shape [n_samples, n_encoded_features]
            The transformed data.

        Returns
        -------
        X_tr : array-like, shape [n_samples, n_features]
            Inverse transformed array.

        """
        # if self._legacy_mode:
        #     raise ValueError("only supported for categorical features")

        check_is_fitted(self, 'categories_')
        X = check_array(X, accept_sparse='csr')

        n_samples, _ = X.shape
        n_features = len(self.categories_)
        n_transformed_features = sum([len(cats) for cats in self.categories_])

        # validate shape of passed X
        msg = ("Shape of the passed X data is not correct. Expected {0} "
               "columns, got {1}.")
        if X.shape[1] != n_transformed_features:
            raise ValueError(msg.format(n_transformed_features, X.shape[1]))

        # create resulting array of appropriate dtype
        dt = np.find_common_type([cat.dtype for cat in self.categories_], [])
        X_tr = np.empty((n_samples, n_features), dtype=dt)

        j = 0
        found_unknown = {}

        for i in range(n_features):
            n_categories = len(self.categories_[i])
            sub = X[:, j:j + n_categories]

            # for sparse X argmax returns 2D matrix, ensure 1D array
            labels = np.asarray(_argmax(sub, axis=1)).flatten()
            X_tr[:, i] = self.categories_[i][labels]

            if self.handle_unknown == 'ignore':
                # ignored unknown categories: we have a row of all zero's
                unknown = np.asarray(sub.sum(axis=1) == 0).flatten()
                if unknown.any():
                    found_unknown[i] = unknown

            j += n_categories

        # if ignored are found: potentially need to upcast result to
        # insert None values
        if found_unknown:
            if X_tr.dtype != object:
                X_tr = X_tr.astype(object)

            for idx, mask in found_unknown.items():
                X_tr[mask, idx] = None

        return X_tr


class OrdinalEncoder(_BaseEncoder):
    """Encode categorical features as an integer array.

    The input to this transformer should be an array-like of integers or
    strings, denoting the values taken on by categorical (discrete) features.
    The features are converted to ordinal integers. This results in
   a single column of integers (0 to n_categories - 1) per feature.

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

    Parameters
    ----------
    categories : 'auto' or a list of lists/arrays of values.
        Categories (unique values) per feature:

        - 'auto' : Determine categories automatically from the training data.
        - list : ``categories[i]`` holds the categories expected in the ith
          column. The passed categories must be sorted and should not mix
          strings and numeric values.

        The used categories can be found in the ``categories_`` attribute.

    dtype : number type, default np.float64
        Desired dtype of output.

    Attributes
    ----------
    categories_ : list of arrays
        The categories of each feature determined during fitting
        (in order corresponding with output of ``transform``).

    Examples
    --------
    Given a dataset with two features, we let the encoder find the unique
    values per feature and transform the data to a binary one-hot encoding.

    >>> from sklearn.preprocessing import OrdinalEncoder
    >>> enc = OrdinalEncoder()
    >>> X = [['Male', 1], ['Female', 3], ['Female', 2]]
    >>> enc.fit(X)
    ... # doctest: +ELLIPSIS
    OrdinalEncoder(categories='auto', dtype=<... 'numpy.float64'>)
    >>> enc.categories_
    [array(['Female', 'Male'], dtype=object), array([1, 2, 3], dtype=object)]
    >>> enc.transform([['Female', 3], ['Male', 1]])
    array([[ 0.,  2.],
           [ 1.,  0.]])

    >>> enc.inverse_transform([[1, 0], [0, 1]])
    array([['Male', 1],
           ['Female', 2]], dtype=object)

    See also
    --------
    sklearn.preprocessing.OneHotEncoder : performs a one-hot encoding of
      categorical features.
    sklearn.preprocessing.LabelEncoder : encodes target labels with values
      between 0 and n_classes-1.
    sklearn.feature_extraction.DictVectorizer : performs a one-hot encoding of
      dictionary items (also handles string-valued features).
    sklearn.feature_extraction.FeatureHasher : performs an approximate one-hot
      encoding of dictionary items or strings.
    """

    def __init__(self, categories='auto', dtype=np.float64):
        self.categories = categories
        self.dtype = dtype

    def fit(self, X, y=None):
        """Fit the OrdinalEncoder to X.

        Parameters
        ----------
        X : array-like, shape [n_samples, n_features]
            The data to determine the categories of each feature.

        Returns
        -------
        self

        """
        self._fit(X)

        return self

    def transform(self, X):
        """Transform X to ordinal codes.

        Parameters
        ----------
        X : array-like, shape [n_samples, n_features]
            The data to encode.

        Returns
        -------
        X_out : sparse matrix or a 2-d array
            Transformed input.

        """
        X_int, _ = self._transform(X)
        return X_int.astype(self.dtype, copy=False)

    def inverse_transform(self, X):
        """Convert back the data to the original representation.

        Parameters
        ----------
        X : array-like or sparse matrix, shape [n_samples, n_encoded_features]
            The transformed data.

        Returns
        -------
        X_tr : array-like, shape [n_samples, n_features]
            Inverse transformed array.

        """
        check_is_fitted(self, 'categories_')
        X = check_array(X, accept_sparse='csr')

        n_samples, _ = X.shape
        n_features = len(self.categories_)

        # validate shape of passed X
        msg = ("Shape of the passed X data is not correct. Expected {0} "
               "columns, got {1}.")
        if X.shape[1] != n_features:
            raise ValueError(msg.format(n_features, X.shape[1]))

        # create resulting array of appropriate dtype
        dt = np.find_common_type([cat.dtype for cat in self.categories_], [])
        X_tr = np.empty((n_samples, n_features), dtype=dt)

        for i in range(n_features):
            labels = X[:, i].astype('int64')
            X_tr[:, i] = self.categories_[i][labels]

        return X_tr