TheAlgorithms/Python · error · ValueError

Valid priorities are 0, 1, and 2

Error message

Valid priorities are 0, 1, and 2

What it means

Raised by FixedPriorityQueue.enqueue() (data_structures/queues/priority_queue_using_list.py:87) when the priority argument indexes outside the self.queues list of exactly three buckets. The implementation catches the resulting IndexError and re-raises it as ValueError with this clearer message, documenting that only priorities 0, 1, 2 exist.

Source

Thrown at data_structures/queues/priority_queue_using_list.py:87

    def __init__(self):
        self.queues = [
            [],
            [],
            [],
        ]

    def enqueue(self, priority: int, data: int) -> None:
        """
        Add an element to a queue based on its priority.
        If the priority is invalid ValueError is raised.
        If the queue is full an OverFlowError is raised.
        """
        try:
            if len(self.queues[priority]) >= 100:
                raise OverflowError("Maximum queue size is 100")
            self.queues[priority].append(data)
        except IndexError:
            raise ValueError("Valid priorities are 0, 1, and 2")

    def dequeue(self) -> int:
        """
        Return the highest priority element in FIFO order.
        If the queue is empty then an under flow exception is raised.
        """
        for queue in self.queues:
            if queue:
                return queue.pop(0)
        raise UnderFlowError("All queues are empty")

    def __str__(self) -> str:
        return "\n".join(f"Priority {i}: {q}" for i, q in enumerate(self.queues))


class ElementPriorityQueue:
    """
    Element Priority Queue is the same as Fixed Priority Queue except that the value of

View on GitHub (pinned to f5988cc097)

Solutions

1. Clamp or map your priorities to 0, 1, 2 before calling enqueue
2. Validate explicitly: if priority not in (0, 1, 2): raise ... with your own message
3. If you need arbitrary priority levels, switch to heapq-based priority queue instead of this fixed 3-bucket demo class

Example fix

// before
pq.enqueue(user_priority, data)  # user_priority=4 -> ValueError

# after
mapped = min(max(user_priority, 0), 2)
pq.enqueue(mapped, data)

if priority not in (0, 1, 2):
    raise ValueError(f'priority {priority} not in 0..2')

def is_valid_priority(p: int) -> bool:
    return isinstance(p, int) and not isinstance(p, bool) and 0 <= p <= 2

try:
    pq.enqueue(priority, data)
except ValueError:
    # re-map or log; note -1 silently wraps, validate BEFORE the call
    priority = min(max(priority, 0), 2)
    pq.enqueue(priority, data)

When it happens

Trigger: enqueue(3, data), enqueue(-4, data), or any priority where self.queues[priority] raises IndexError. Beware: negative indices like -1 do NOT raise IndexError (they index from the end), so priority=-1 silently writes to bucket 2 instead of erroring.

Common situations: Mapping external priority schemes (1-5, 0-10, or 'high'/'low' strings) directly onto this class, off-by-one after converting from a 1-based priority system, or assuming negative priority means 'lowest'.

Related errors

• Maximum queue size is 100
• All queues are empty
• The queue is empty
• number must be positive
• The value of input must be non-negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/44ad782e3f7e1fa3. Report an issue: GitHub.