transformer.py

# Authors: Mainak Jas <mainak@neuro.hut.fi>
#          Alexandre Gramfort <alexandre.gramfort@inria.fr>
#          Romain Trachel <trachelr@gmail.com>
#
# License: BSD-3-Clause

import numpy as np

from .mixin import TransformerMixin
from .base import BaseEstimator

from .. import pick_types
from ..filter import filter_data
from ..time_frequency import psd_array_multitaper
from ..utils import fill_doc, _check_option, _validate_type, verbose
from ..io.pick import pick_info, _pick_data_channels, _picks_by_type, _picks_to_idx
from ..cov import _check_scalings_user


class _ConstantScaler:
    """Scale channel types using constant values."""

    def __init__(self, info, scalings, do_scaling=True):
        self._scalings = scalings
        self._info = info
        self._do_scaling = do_scaling

    def fit(self, X, y=None):
        scalings = _check_scalings_user(self._scalings)
        picks_by_type = _picks_by_type(
            pick_info(self._info, _pick_data_channels(self._info, exclude=()))
        )
        std = np.ones(sum(len(p[1]) for p in picks_by_type))
        if X.shape[1] != len(std):
            raise ValueError(
                "info had %d data channels but X has %d channels" % (len(std), len(X))
            )
        if self._do_scaling:  # this is silly, but necessary for completeness
            for kind, picks in picks_by_type:
                std[picks] = 1.0 / scalings[kind]
        self.std_ = std
        self.mean_ = np.zeros_like(std)
        return self

    def transform(self, X):
        return X / self.std_

    def inverse_transform(self, X, y=None):
        return X * self.std_

    def fit_transform(self, X, y=None):
        return self.fit(X, y).transform(X)


def _sklearn_reshape_apply(func, return_result, X, *args, **kwargs):
    """Reshape epochs and apply function."""
    if not isinstance(X, np.ndarray):
        raise ValueError("data should be an np.ndarray, got %s." % type(X))
    orig_shape = X.shape
    X = np.reshape(X.transpose(0, 2, 1), (-1, orig_shape[1]))
    X = func(X, *args, **kwargs)
    if return_result:
        X.shape = (orig_shape[0], orig_shape[2], orig_shape[1])
        X = X.transpose(0, 2, 1)
        return X


@fill_doc
class Scaler(TransformerMixin, BaseEstimator):
    """Standardize channel data.

    This class scales data for each channel. It differs from scikit-learn
    classes (e.g., :class:`sklearn.preprocessing.StandardScaler`) in that
    it scales each *channel* by estimating μ and σ using data from all
    time points and epochs, as opposed to standardizing each *feature*
    (i.e., each time point for each channel) by estimating using μ and σ
    using data from all epochs.

    Parameters
    ----------
    %(info)s Only necessary if ``scalings`` is a dict or None.
    scalings : dict, str, default None
        Scaling method to be applied to data channel wise.

        * if scalings is None (default), scales mag by 1e15, grad by 1e13,
          and eeg by 1e6.
        * if scalings is :class:`dict`, keys are channel types and values
          are scale factors.
        * if ``scalings=='median'``,
          :class:`sklearn.preprocessing.RobustScaler`
          is used (requires sklearn version 0.17+).
        * if ``scalings=='mean'``,
          :class:`sklearn.preprocessing.StandardScaler`
          is used.

    with_mean : bool, default True
        If True, center the data using mean (or median) before scaling.
        Ignored for channel-type scaling.
    with_std : bool, default True
        If True, scale the data to unit variance (``scalings='mean'``),
        quantile range (``scalings='median``), or using channel type
        if ``scalings`` is a dict or None).
    """

    def __init__(
        self, info=None, scalings=None, with_mean=True, with_std=True
    ):  # noqa: D102
        self.info = info
        self.with_mean = with_mean
        self.with_std = with_std
        self.scalings = scalings

        if not (scalings is None or isinstance(scalings, (dict, str))):
            raise ValueError(
                "scalings type should be dict, str, or None, " "got %s" % type(scalings)
            )
        if isinstance(scalings, str):
            _check_option("scalings", scalings, ["mean", "median"])
        if scalings is None or isinstance(scalings, dict):
            if info is None:
                raise ValueError(
                    'Need to specify "info" if scalings is' "%s" % type(scalings)
                )
            self._scaler = _ConstantScaler(info, scalings, self.with_std)
        elif scalings == "mean":
            from sklearn.preprocessing import StandardScaler

            self._scaler = StandardScaler(
                with_mean=self.with_mean, with_std=self.with_std
            )
        else:  # scalings == 'median':
            from sklearn.preprocessing import RobustScaler

            self._scaler = RobustScaler(
                with_centering=self.with_mean, with_scaling=self.with_std
            )

    def fit(self, epochs_data, y=None):
        """Standardize data across channels.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data to concatenate channels.
        y : array, shape (n_epochs,)
            The label for each epoch.

        Returns
        -------
        self : instance of Scaler
            The modified instance.
        """
        _validate_type(epochs_data, np.ndarray, "epochs_data")
        if epochs_data.ndim == 2:
            epochs_data = epochs_data[..., np.newaxis]
        assert epochs_data.ndim == 3, epochs_data.shape
        _sklearn_reshape_apply(self._scaler.fit, False, epochs_data, y=y)
        return self

    def transform(self, epochs_data):
        """Standardize data across channels.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels[, n_times])
            The data.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The data concatenated over channels.

        Notes
        -----
        This function makes a copy of the data before the operations and the
        memory usage may be large with big data.
        """
        _validate_type(epochs_data, np.ndarray, "epochs_data")
        if epochs_data.ndim == 2:  # can happen with SlidingEstimator
            if self.info is not None:
                assert len(self.info["ch_names"]) == epochs_data.shape[1]
            epochs_data = epochs_data[..., np.newaxis]
        assert epochs_data.ndim == 3, epochs_data.shape
        return _sklearn_reshape_apply(self._scaler.transform, True, epochs_data)

    def fit_transform(self, epochs_data, y=None):
        """Fit to data, then transform it.

        Fits transformer to epochs_data and y and returns a transformed version
        of epochs_data.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.
        y : None | array, shape (n_epochs,)
            The label for each epoch.
            Defaults to None.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The data concatenated over channels.

        Notes
        -----
        This function makes a copy of the data before the operations and the
        memory usage may be large with big data.
        """
        return self.fit(epochs_data, y).transform(epochs_data)

    def inverse_transform(self, epochs_data):
        """Invert standardization of data across channels.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The data concatenated over channels.

        Notes
        -----
        This function makes a copy of the data before the operations and the
        memory usage may be large with big data.
        """
        assert epochs_data.ndim == 3, epochs_data.shape
        return _sklearn_reshape_apply(self._scaler.inverse_transform, True, epochs_data)


class Vectorizer(TransformerMixin):
    """Transform n-dimensional array into 2D array of n_samples by n_features.

    This class reshapes an n-dimensional array into an n_samples * n_features
    array, usable by the estimators and transformers of scikit-learn.

    Attributes
    ----------
    features_shape_ : tuple
         Stores the original shape of data.

    Examples
    --------
    clf = make_pipeline(SpatialFilter(), _XdawnTransformer(), Vectorizer(),
                        LogisticRegression())
    """

    def fit(self, X, y=None):
        """Store the shape of the features of X.

        Parameters
        ----------
        X : array-like
            The data to fit. Can be, for example a list, or an array of at
            least 2d. The first dimension must be of length n_samples, where
            samples are the independent samples used by the estimator
            (e.g. n_epochs for epoched data).
        y : None | array, shape (n_samples,)
            Used for scikit-learn compatibility.

        Returns
        -------
        self : instance of Vectorizer
            Return the modified instance.
        """
        X = np.asarray(X)
        self.features_shape_ = X.shape[1:]
        return self

    def transform(self, X):
        """Convert given array into two dimensions.

        Parameters
        ----------
        X : array-like
            The data to fit. Can be, for example a list, or an array of at
            least 2d. The first dimension must be of length n_samples, where
            samples are the independent samples used by the estimator
            (e.g. n_epochs for epoched data).

        Returns
        -------
        X : array, shape (n_samples, n_features)
            The transformed data.
        """
        X = np.asarray(X)
        if X.shape[1:] != self.features_shape_:
            raise ValueError("Shape of X used in fit and transform must be " "same")
        return X.reshape(len(X), -1)

    def fit_transform(self, X, y=None):
        """Fit the data, then transform in one step.

        Parameters
        ----------
        X : array-like
            The data to fit. Can be, for example a list, or an array of at
            least 2d. The first dimension must be of length n_samples, where
            samples are the independent samples used by the estimator
            (e.g. n_epochs for epoched data).
        y : None | array, shape (n_samples,)
            Used for scikit-learn compatibility.

        Returns
        -------
        X : array, shape (n_samples, -1)
            The transformed data.
        """
        return self.fit(X).transform(X)

    def inverse_transform(self, X):
        """Transform 2D data back to its original feature shape.

        Parameters
        ----------
        X : array-like, shape (n_samples,  n_features)
            Data to be transformed back to original shape.

        Returns
        -------
        X : array
            The data transformed into shape as used in fit. The first
            dimension is of length n_samples.
        """
        X = np.asarray(X)
        if X.ndim not in (2, 3):
            raise ValueError(
                "X should be of 2 or 3 dimensions but has shape " "%s" % (X.shape,)
            )
        return X.reshape(X.shape[:-1] + self.features_shape_)


@fill_doc
class PSDEstimator(TransformerMixin):
    """Compute power spectral density (PSD) using a multi-taper method.

    Parameters
    ----------
    sfreq : float
        The sampling frequency.
    fmin : float
        The lower frequency of interest.
    fmax : float
        The upper frequency of interest.
    bandwidth : float
        The bandwidth of the multi taper windowing function in Hz.
    adaptive : bool
        Use adaptive weights to combine the tapered spectra into PSD
        (slow, use n_jobs >> 1 to speed up computation).
    low_bias : bool
        Only use tapers with more than 90%% spectral concentration within
        bandwidth.
    n_jobs : int
        Number of parallel jobs to use (only used if adaptive=True).
    %(normalization)s
    %(verbose)s

    See Also
    --------
    mne.time_frequency.psd_array_multitaper
    mne.io.Raw.compute_psd
    mne.Epochs.compute_psd
    mne.Evoked.compute_psd
    """

    @verbose
    def __init__(
        self,
        sfreq=2 * np.pi,
        fmin=0,
        fmax=np.inf,
        bandwidth=None,
        adaptive=False,
        low_bias=True,
        n_jobs=None,
        normalization="length",
        *,
        verbose=None
    ):  # noqa: D102
        self.sfreq = sfreq
        self.fmin = fmin
        self.fmax = fmax
        self.bandwidth = bandwidth
        self.adaptive = adaptive
        self.low_bias = low_bias
        self.n_jobs = n_jobs
        self.normalization = normalization

    def fit(self, epochs_data, y):
        """Compute power spectral density (PSD) using a multi-taper method.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.
        y : array, shape (n_epochs,)
            The label for each epoch.

        Returns
        -------
        self : instance of PSDEstimator
            The modified instance.
        """
        if not isinstance(epochs_data, np.ndarray):
            raise ValueError(
                "epochs_data should be of type ndarray (got %s)." % type(epochs_data)
            )

        return self

    def transform(self, epochs_data):
        """Compute power spectral density (PSD) using a multi-taper method.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.

        Returns
        -------
        psd : array, shape (n_signals, n_freqs) or (n_freqs,)
            The computed PSD.
        """
        if not isinstance(epochs_data, np.ndarray):
            raise ValueError(
                "epochs_data should be of type ndarray (got %s)." % type(epochs_data)
            )
        psd, _ = psd_array_multitaper(
            epochs_data,
            sfreq=self.sfreq,
            fmin=self.fmin,
            fmax=self.fmax,
            bandwidth=self.bandwidth,
            adaptive=self.adaptive,
            low_bias=self.low_bias,
            normalization=self.normalization,
            n_jobs=self.n_jobs,
        )
        return psd


@fill_doc
class FilterEstimator(TransformerMixin):
    """Estimator to filter RtEpochs.

    Applies a zero-phase low-pass, high-pass, band-pass, or band-stop
    filter to the channels selected by "picks".

    l_freq and h_freq are the frequencies below which and above which,
    respectively, to filter out of the data. Thus the uses are:

        - l_freq < h_freq: band-pass filter
        - l_freq > h_freq: band-stop filter
        - l_freq is not None, h_freq is None: low-pass filter
        - l_freq is None, h_freq is not None: high-pass filter

    If n_jobs > 1, more memory is required as "len(picks) * n_times"
    additional time points need to be temporarily stored in memory.

    Parameters
    ----------
    %(info_not_none)s
    %(l_freq)s
    %(h_freq)s
    %(picks_good_data)s
    %(filter_length)s
    %(l_trans_bandwidth)s
    %(h_trans_bandwidth)s
    n_jobs : int | str
        Number of jobs to run in parallel.
        Can be 'cuda' if ``cupy`` is installed properly and method='fir'.
    method : str
        'fir' will use overlap-add FIR filtering, 'iir' will use IIR filtering.
    iir_params : dict | None
        Dictionary of parameters to use for IIR filtering.
        See mne.filter.construct_iir_filter for details. If iir_params
        is None and method="iir", 4th order Butterworth will be used.
    %(fir_design)s
    %(verbose)s

    See Also
    --------
    TemporalFilter

    Notes
    -----
    This is primarily meant for use in conjunction with
    :class:`mne_realtime.RtEpochs`. In general it is not recommended in a
    normal processing pipeline as it may result in edge artifacts. Use with
    caution.
    """

    def __init__(
        self,
        info,
        l_freq,
        h_freq,
        picks=None,
        filter_length="auto",
        l_trans_bandwidth="auto",
        h_trans_bandwidth="auto",
        n_jobs=None,
        method="fir",
        iir_params=None,
        fir_design="firwin",
        *,
        verbose=None
    ):  # noqa: D102
        self.info = info
        self.l_freq = l_freq
        self.h_freq = h_freq
        self.picks = _picks_to_idx(info, picks)
        self.filter_length = filter_length
        self.l_trans_bandwidth = l_trans_bandwidth
        self.h_trans_bandwidth = h_trans_bandwidth
        self.n_jobs = n_jobs
        self.method = method
        self.iir_params = iir_params
        self.fir_design = fir_design

    def fit(self, epochs_data, y):
        """Filter data.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.
        y : array, shape (n_epochs,)
            The label for each epoch.

        Returns
        -------
        self : instance of FilterEstimator
            The modified instance.
        """
        if not isinstance(epochs_data, np.ndarray):
            raise ValueError(
                "epochs_data should be of type ndarray (got %s)." % type(epochs_data)
            )

        if self.picks is None:
            self.picks = pick_types(
                self.info, meg=True, eeg=True, ref_meg=False, exclude=[]
            )

        if self.l_freq == 0:
            self.l_freq = None
        if self.h_freq is not None and self.h_freq > (self.info["sfreq"] / 2.0):
            self.h_freq = None
        if self.l_freq is not None and not isinstance(self.l_freq, float):
            self.l_freq = float(self.l_freq)
        if self.h_freq is not None and not isinstance(self.h_freq, float):
            self.h_freq = float(self.h_freq)

        if self.info["lowpass"] is None or (
            self.h_freq is not None
            and (self.l_freq is None or self.l_freq < self.h_freq)
            and self.h_freq < self.info["lowpass"]
        ):
            with self.info._unlock():
                self.info["lowpass"] = self.h_freq

        if self.info["highpass"] is None or (
            self.l_freq is not None
            and (self.h_freq is None or self.l_freq < self.h_freq)
            and self.l_freq > self.info["highpass"]
        ):
            with self.info._unlock():
                self.info["highpass"] = self.l_freq

        return self

    def transform(self, epochs_data):
        """Filter data.

        Parameters
        ----------
        epochs_data : array, shape (n_epochs, n_channels, n_times)
            The data.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The data after filtering.
        """
        if not isinstance(epochs_data, np.ndarray):
            raise ValueError(
                "epochs_data should be of type ndarray (got %s)." % type(epochs_data)
            )
        epochs_data = np.atleast_3d(epochs_data)
        return filter_data(
            epochs_data,
            self.info["sfreq"],
            self.l_freq,
            self.h_freq,
            self.picks,
            self.filter_length,
            self.l_trans_bandwidth,
            self.h_trans_bandwidth,
            method=self.method,
            iir_params=self.iir_params,
            n_jobs=self.n_jobs,
            copy=False,
            fir_design=self.fir_design,
            verbose=False,
        )


class UnsupervisedSpatialFilter(TransformerMixin, BaseEstimator):
    """Use unsupervised spatial filtering across time and samples.

    Parameters
    ----------
    estimator : instance of sklearn.base.BaseEstimator
        Estimator using some decomposition algorithm.
    average : bool, default False
        If True, the estimator is fitted on the average across samples
        (e.g. epochs).
    """

    def __init__(self, estimator, average=False):  # noqa: D102
        # XXX: Use _check_estimator #3381
        for attr in ("fit", "transform", "fit_transform"):
            if not hasattr(estimator, attr):
                raise ValueError(
                    "estimator must be a scikit-learn "
                    "transformer, missing %s method" % attr
                )

        if not isinstance(average, bool):
            raise ValueError(
                "average parameter must be of bool type, got " "%s instead" % type(bool)
            )

        self.estimator = estimator
        self.average = average

    def fit(self, X, y=None):
        """Fit the spatial filters.

        Parameters
        ----------
        X : array, shape (n_epochs, n_channels, n_times)
            The data to be filtered.
        y : None | array, shape (n_samples,)
            Used for scikit-learn compatibility.

        Returns
        -------
        self : instance of UnsupervisedSpatialFilter
            Return the modified instance.
        """
        if self.average:
            X = np.mean(X, axis=0).T
        else:
            n_epochs, n_channels, n_times = X.shape
            # trial as time samples
            X = np.transpose(X, (1, 0, 2)).reshape((n_channels, n_epochs * n_times)).T
        self.estimator.fit(X)
        return self

    def fit_transform(self, X, y=None):
        """Transform the data to its filtered components after fitting.

        Parameters
        ----------
        X : array, shape (n_epochs, n_channels, n_times)
            The data to be filtered.
        y : None | array, shape (n_samples,)
            Used for scikit-learn compatibility.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The transformed data.
        """
        return self.fit(X).transform(X)

    def transform(self, X):
        """Transform the data to its spatial filters.

        Parameters
        ----------
        X : array, shape (n_epochs, n_channels, n_times)
            The data to be filtered.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The transformed data.
        """
        return self._apply_method(X, "transform")

    def inverse_transform(self, X):
        """Inverse transform the data to its original space.

        Parameters
        ----------
        X : array, shape (n_epochs, n_components, n_times)
            The data to be inverted.

        Returns
        -------
        X : array, shape (n_epochs, n_channels, n_times)
            The transformed data.
        """
        return self._apply_method(X, "inverse_transform")

    def _apply_method(self, X, method):
        """Vectorize time samples as trials, apply method and reshape back.

        Parameters
        ----------
        X : array, shape (n_epochs, n_dims, n_times)
            The data to be inverted.

        Returns
        -------
        X : array, shape (n_epochs, n_dims, n_times)
            The transformed data.
        """
        n_epochs, n_channels, n_times = X.shape
        # trial as time samples
        X = np.transpose(X, [1, 0, 2])
        X = np.reshape(X, [n_channels, n_epochs * n_times]).T
        # apply method
        method = getattr(self.estimator, method)
        X = method(X)
        # put it back to n_epochs, n_dimensions
        X = np.reshape(X.T, [-1, n_epochs, n_times]).transpose([1, 0, 2])
        return X


@fill_doc
class TemporalFilter(TransformerMixin):
    """Estimator to filter data array along the last dimension.

    Applies a zero-phase low-pass, high-pass, band-pass, or band-stop
    filter to the channels.

    l_freq and h_freq are the frequencies below which and above which,
    respectively, to filter out of the data. Thus the uses are:

        - l_freq < h_freq: band-pass filter
        - l_freq > h_freq: band-stop filter
        - l_freq is not None, h_freq is None: low-pass filter
        - l_freq is None, h_freq is not None: high-pass filter

    See :func:`mne.filter.filter_data`.

    Parameters
    ----------
    l_freq : float | None
        Low cut-off frequency in Hz. If None the data are only low-passed.
    h_freq : float | None
        High cut-off frequency in Hz. If None the data are only
        high-passed.
    sfreq : float, default 1.0
        Sampling frequency in Hz.
    filter_length : str | int, default 'auto'
        Length of the FIR filter to use (if applicable):

            * int: specified length in samples.
            * 'auto' (default in 0.14): the filter length is chosen based
              on the size of the transition regions (7 times the reciprocal
              of the shortest transition band).
            * str: (default in 0.13 is "10s") a human-readable time in
              units of "s" or "ms" (e.g., "10s" or "5500ms") will be
              converted to that number of samples if ``phase="zero"``, or
              the shortest power-of-two length at least that duration for
              ``phase="zero-double"``.

    l_trans_bandwidth : float | str
        Width of the transition band at the low cut-off frequency in Hz
        (high pass or cutoff 1 in bandpass). Can be "auto"
        (default in 0.14) to use a multiple of ``l_freq``::

            min(max(l_freq * 0.25, 2), l_freq)

        Only used for ``method='fir'``.
    h_trans_bandwidth : float | str
        Width of the transition band at the high cut-off frequency in Hz
        (low pass or cutoff 2 in bandpass). Can be "auto"
        (default in 0.14) to use a multiple of ``h_freq``::

            min(max(h_freq * 0.25, 2.), info['sfreq'] / 2. - h_freq)

        Only used for ``method='fir'``.
    n_jobs : int | str, default 1
        Number of jobs to run in parallel.
        Can be 'cuda' if ``cupy`` is installed properly and method='fir'.
    method : str, default 'fir'
        'fir' will use overlap-add FIR filtering, 'iir' will use IIR
        forward-backward filtering (via filtfilt).
    iir_params : dict | None, default None
        Dictionary of parameters to use for IIR filtering.
        See mne.filter.construct_iir_filter for details. If iir_params
        is None and method="iir", 4th order Butterworth will be used.
    fir_window : str, default 'hamming'
        The window to use in FIR design, can be "hamming", "hann",
        or "blackman".
    fir_design : str
        Can be "firwin" (default) to use :func:`scipy.signal.firwin`,
        or "firwin2" to use :func:`scipy.signal.firwin2`. "firwin" uses
        a time-domain design technique that generally gives improved
        attenuation using fewer samples than "firwin2".

        .. versionadded:: 0.15
    %(verbose)s

    See Also
    --------
    FilterEstimator
    Vectorizer
    mne.filter.filter_data
    """

    @verbose
    def __init__(
        self,
        l_freq=None,
        h_freq=None,
        sfreq=1.0,
        filter_length="auto",
        l_trans_bandwidth="auto",
        h_trans_bandwidth="auto",
        n_jobs=None,
        method="fir",
        iir_params=None,
        fir_window="hamming",
        fir_design="firwin",
        *,
        verbose=None
    ):  # noqa: D102
        self.l_freq = l_freq
        self.h_freq = h_freq
        self.sfreq = sfreq
        self.filter_length = filter_length
        self.l_trans_bandwidth = l_trans_bandwidth
        self.h_trans_bandwidth = h_trans_bandwidth
        self.n_jobs = n_jobs
        self.method = method
        self.iir_params = iir_params
        self.fir_window = fir_window
        self.fir_design = fir_design

        if not isinstance(self.n_jobs, int) and self.n_jobs == "cuda":
            raise ValueError(
                'n_jobs must be int or "cuda", got %s instead.' % type(self.n_jobs)
            )

    def fit(self, X, y=None):
        """Do nothing (for scikit-learn compatibility purposes).

        Parameters
        ----------
        X : array, shape (n_epochs, n_channels, n_times) or or shape (n_channels, n_times)
            The data to be filtered over the last dimension. The channels
            dimension can be zero when passing a 2D array.
        y : None
            Not used, for scikit-learn compatibility issues.

        Returns
        -------
        self : instance of TemporalFilter
            The modified instance.
        """  # noqa: E501
        return self

    def transform(self, X):
        """Filter data along the last dimension.

        Parameters
        ----------
        X : array, shape (n_epochs, n_channels, n_times) or shape (n_channels, n_times)
            The data to be filtered over the last dimension. The channels
            dimension can be zero when passing a 2D array.

        Returns
        -------
        X : array
            The data after filtering.
        """  # noqa: E501
        X = np.atleast_2d(X)

        if X.ndim > 3:
            raise ValueError(
                "Array must be of at max 3 dimensions instead "
                "got %s dimensional matrix" % (X.ndim)
            )

        shape = X.shape
        X = X.reshape(-1, shape[-1])
        X = filter_data(
            X,
            self.sfreq,
            self.l_freq,
            self.h_freq,
            filter_length=self.filter_length,
            l_trans_bandwidth=self.l_trans_bandwidth,
            h_trans_bandwidth=self.h_trans_bandwidth,
            n_jobs=self.n_jobs,
            method=self.method,
            iir_params=self.iir_params,
            copy=False,
            fir_window=self.fir_window,
            fir_design=self.fir_design,
        )
        return X.reshape(shape)