def get_portgroup_by_id(task, portgroup_id):
    """Lookup a portgroup by ID on a task object.

    :param task: a TaskManager instance
    :param portgroup_id: ID of the portgroup.
    :returns: A Portgroup object or None.
    """
    for portgroup in task.portgroups:
        if portgroup.id == portgroup_id:
            return portgroup