def r2_score(y_true, y_pred,
             sample_weight=None,
             multioutput=None):
    """R^2 (coefficient of determination) regression score function.

    Best possible score is 1.0 and it can be negative (because the
    model can be arbitrarily worse). A constant model that always
    predicts the expected value of y, disregarding the input features,
    would get a R^2 score of 0.0.

    Read more in the :ref:`User Guide <r2_score>`.

    Parameters
    ----------
    y_true : array-like of shape = (n_samples) or (n_samples, n_outputs)
        Ground truth (correct) target values.

    y_pred : array-like of shape = (n_samples) or (n_samples, n_outputs)
        Estimated target values.

    sample_weight : array-like of shape = (n_samples), optional
        Sample weights.

    multioutput : string in ['raw_values', 'uniform_average', \
'variance_weighted'] or None or array-like of shape (n_outputs)

        Defines aggregating of multiple output scores.
        Array-like value defines weights used to average scores.
        Default value corresponds to 'variance_weighted', this behaviour is
        deprecated since version 0.17 and will be changed to 'uniform_average'
        starting from 0.19.

        'raw_values' :
            Returns a full set of scores in case of multioutput input.

        'uniform_average' :
            Scores of all outputs are averaged with uniform weight.

        'variance_weighted' :
            Scores of all outputs are averaged, weighted by the variances
            of each individual output.

    Returns
    -------
    z : float or ndarray of floats
        The R^2 score or ndarray of scores if 'multioutput' is
        'raw_values'.

    Notes
    -----
    This is not a symmetric function.

    Unlike most other scores, R^2 score may be negative (it need not actually
    be the square of a quantity R).

    References
    ----------
    .. [1] `Wikipedia entry on the Coefficient of determination
            <https://en.wikipedia.org/wiki/Coefficient_of_determination>`_

    Examples
    --------
    >>> from sklearn.metrics import r2_score
    >>> y_true = [3, -0.5, 2, 7]
    >>> y_pred = [2.5, 0.0, 2, 8]
    >>> r2_score(y_true, y_pred)  # doctest: +ELLIPSIS
    0.948...
    >>> y_true = [[0.5, 1], [-1, 1], [7, -6]]
    >>> y_pred = [[0, 2], [-1, 2], [8, -5]]
    >>> r2_score(y_true, y_pred, multioutput='variance_weighted')  # doctest: +ELLIPSIS
    0.938...

    """
    y_type, y_true, y_pred, multioutput = _check_reg_targets(
        y_true, y_pred, multioutput)

    if sample_weight is not None:
        sample_weight = column_or_1d(sample_weight)
        weight = sample_weight[:, np.newaxis]
    else:
        weight = 1.

    numerator = (weight * (y_true - y_pred) ** 2).sum(axis=0,
                                                      dtype=np.float64)
    denominator = (weight * (y_true - np.average(
        y_true, axis=0, weights=sample_weight)) ** 2).sum(axis=0,
                                                          dtype=np.float64)
    nonzero_denominator = denominator != 0
    nonzero_numerator = numerator > 0
    valid_score = nonzero_denominator & nonzero_numerator
    output_scores = np.ones([y_true.shape[1]])
    output_scores[valid_score] = 1 - (numerator[valid_score] /
                                      denominator[valid_score])
    # arbitrary set to zero to avoid -inf scores, having a constant
    # y_true is not interesting for scoring a regression anyway
    output_scores[nonzero_numerator & ~nonzero_denominator] = 0.
    if multioutput is None and y_true.shape[1] != 1:
        warnings.warn("Default 'multioutput' behavior now corresponds to "
                      "'variance_weighted' value which is deprecated since "
                      "0.17, it will be changed to 'uniform_average' "
                      "starting from 0.19.",
                      DeprecationWarning)
        multioutput = 'variance_weighted'
    if isinstance(multioutput, string_types):
        if multioutput == 'raw_values':
            # return scores individually
            return output_scores
        elif multioutput == 'uniform_average':
            # passing None as weights results is uniform mean
            avg_weights = None
        elif multioutput == 'variance_weighted':
            avg_weights = denominator
            # avoid fail on constant y or one-element arrays
            if not np.any(nonzero_denominator):
                if not np.any(nonzero_numerator):
                    return 1.0
                else:
                    return 0.0
    else:
        avg_weights = multioutput

    return np.average(output_scores, weights=avg_weights)
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