Push the item in the priority queue.
        if priority is not given, priority is set to the value of item.

def push(self, item, priority=None):
        """Push the item in the priority queue.
        if priority is not given, priority is set to the value of item.
        """
        priority = item if priority is None else priority
        node = PriorityQueueNode(item, priority)
        for index, current in enumerate(self.priority_queue_list):
            if current.priority < node.priority:
                self.priority_queue_list.insert(index, node)
                return
        # when traversed complete queue
        self.priority_queue_list.append(node)