TheAlgorithms/Python · error · UnderFlowError

The queue is empty

Error message

The queue is empty

What it means

Raised by ElementPriorityQueue.dequeue() (data_structures/queues/priority_queue_using_list.py:167) when self.queue is empty. It uses the module's custom UnderFlowError, mirroring FixedPriorityQueue's underflow behavior. The normal path pops the minimum-valued element (value = priority).

Source

Thrown at data_structures/queues/priority_queue_using_list.py:167

    def __init__(self):
        self.queue = []

    def enqueue(self, data: int) -> None:
        """
        This function enters the element into the queue
        If the queue is full an Exception is raised saying Over Flow!
        """
        if len(self.queue) == 100:
            raise OverFlowError("Maximum queue size is 100")
        self.queue.append(data)

    def dequeue(self) -> int:
        """
        Return the highest priority element in FIFO order.
        If the queue is empty then an under flow exception is raised.
        """
        if not self.queue:
            raise UnderFlowError("The queue is empty")
        else:
            data = min(self.queue)
            self.queue.remove(data)
            return data

    def __str__(self) -> str:
        """
        Prints all the elements within the Element Priority Queue
        """
        return str(self.queue)


def fixed_priority_queue():
    fpq = FixedPriorityQueue()
    fpq.enqueue(0, 10)
    fpq.enqueue(1, 70)
    fpq.enqueue(0, 100)
    fpq.enqueue(2, 1)

View on GitHub (pinned to f5988cc097)

Solutions

1. Guard with `if pq.queue:` before calling dequeue()
2. Catch UnderFlowError imported from data_structures.queues.priority_queue_using_list
3. Loop with `while len(pq.queue):` so the loop condition and the dequeue share the same state

Example fix

// before
smallest = pq.dequeue()  # UnderFlowError on empty

# after
smallest = pq.dequeue() if pq.queue else None

item = pq.dequeue() if pq.queue else None

from data_structures.queues.priority_queue_using_list import UnderFlowError
try:
    item = pq.dequeue()
except UnderFlowError:
    item = None

When it happens

Trigger: dequeue() on a new ElementPriorityQueue, or after the 100th... after all enqueued elements have been dequeued (list is empty).

Common situations: Drain loops (while True: pq.dequeue()), or size checks that race with another consumer of the same queue object.

Related errors

• All queues are empty
• Maximum queue size is 100
• Valid priorities are 0, 1, and 2
• UNDERFLOW
• dequeue from empty queue

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/dab6fa3e328b8c4a. Report an issue: GitHub.