def sample_batch_indexes(low, high, size):
    """Return a sample of (size) unique elements between low and high

        # Argument
            low (int): The minimum value for our samples
            high (int): The maximum value for our samples
            size (int): The number of samples to pick

        # Returns
            A list of samples of length size, with values between low and high
        """
    if high - low >= size:
        # We have enough data. Draw without replacement, that is each index is unique in the
        # batch. We cannot use `np.random.choice` here because it is horribly inefficient as
        # the memory grows. See https://github.com/numpy/numpy/issues/2764 for a discussion.
        # `random.sample` does the same thing (drawing without replacement) and is way faster.
        try:
            r = xrange(low, high)
        except NameError:
            r = range(low, high)
        batch_idxs = random.sample(r, size)
    else:
        # Not enough data. Help ourselves with sampling from the range, but the same index
        # can occur multiple times. This is not good and should be avoided by picking a
        # large enough warm-up phase.
        warnings.warn('Not enough entries to sample without replacement. Consider increasing your warm-up phase to avoid oversampling!')
        batch_idxs = np.random.random_integers(low, high - 1, size=size)
    assert len(batch_idxs) == size
    return batch_idxs