def identity(X):
    """Simply return the input array.

    Parameters
    ----------
    X : {array-like, sparse matrix}, shape (n_samples, n_features)
        Data, where n_samples is the number of samples
        and n_features is the number of features.

    Returns
    -------
    X : {array-like, sparse matrix}, shape (n_samples, n_features)
        Same as the input data.
    """
    return X