human
Complete this python program:
def batch(states, batch_size=None):
assistant
#Here is the Python code you requested.

    """Combines a collection of state structures into a batch, padding if needed.
    Args:
    states: A collection of individual nested state structures.
    batch_size: The desired final batch size. If the nested state structure
    that results from combining the states is smaller than this, it will be
    padded with zeros.
    Returns:
    A single state structure that results from stacking the structures in
    `states`, with padding if needed.
    Raises:
    ValueError: If the number of input states is larger than `batch_size`.
    """
    if (batch_size and (len(states) > batch_size)):
        raise ValueError('Combined    state    is    larger    than    the    requested    batch    size')
    def stack_and_pad(*states):
        stacked = np.stack(states)
        if batch_size:
            stacked.resize(([batch_size] + list(stacked.shape)[1:]))
        return stacked
    return tf_nest.map_structure(stack_and_pad, *states)