def contrastive_loss(left, right, y, margin, extra=False, scope="constrastive_loss"):
    r"""Loss for Siamese networks as described in the paper:
    `Learning a Similarity Metric Discriminatively, with Application to Face
    Verification <http://yann.lecun.com/exdb/publis/pdf/chopra-05.pdf>`_ by Chopra et al.

    .. math::
        \frac{1}{2} [y \cdot d^2 + (1-y) \cdot \max(0, m - d)^2], d = \Vert l - r \Vert_2

    Args:
        left (tf.Tensor): left feature vectors of shape [Batch, N].
        right (tf.Tensor): right feature vectors of shape [Batch, N].
        y (tf.Tensor): binary labels of shape [Batch]. 1: similar, 0: not similar.
        margin (float): horizon for negative examples (y==0).
        extra (bool): also return distances for pos and neg.

    Returns:
        tf.Tensor: constrastive_loss (averaged over the batch), (and optionally average_pos_dist, average_neg_dist)
    """
    with tf.name_scope(scope):
        y = tf.cast(y, tf.float32)

        delta = tf.reduce_sum(tf.square(left - right), 1)
        delta_sqrt = tf.sqrt(delta + 1e-10)

        match_loss = delta
        missmatch_loss = tf.square(tf.nn.relu(margin - delta_sqrt))

        loss = tf.reduce_mean(0.5 * (y * match_loss + (1 - y) * missmatch_loss))

        if extra:
            num_pos = tf.count_nonzero(y)
            num_neg = tf.count_nonzero(1 - y)
            pos_dist = tf.where(tf.equal(num_pos, 0), 0.,
                                tf.reduce_sum(y * delta_sqrt) / tf.cast(num_pos, tf.float32),
                                name="pos-dist")
            neg_dist = tf.where(tf.equal(num_neg, 0), 0.,
                                tf.reduce_sum((1 - y) * delta_sqrt) / tf.cast(num_neg, tf.float32),
                                name="neg-dist")
            return loss, pos_dist, neg_dist
        else:
            return loss
r"""Loss for Siamese networks as described in the paper:
    `Learning a Similarity Metric Discriminatively, with Application to Face
    Verification <http://yann.lecun.com/exdb/publis/pdf/chopra-05.pdf>`_ by Chopra et al.

    .. math::
        \frac{1}{2} [y \cdot d^2 + (1-y) \cdot \max(0, m - d)^2], d = \Vert l - r \Vert_2

    Args:
        left (tf.Tensor): left feature vectors of shape [Batch, N].
        right (tf.Tensor): right feature vectors of shape [Batch, N].
        y (tf.Tensor): binary labels of shape [Batch]. 1: similar, 0: not similar.
        margin (float): horizon for negative examples (y==0).
        extra (bool): also return distances for pos and neg.

    Returns:
        tf.Tensor: constrastive_loss (averaged over the batch), (and optionally average_pos_dist, average_neg_dist)