TheAlgorithms/Python · error · IndexError

Queue is empty

Error message

Queue is empty

What it means

Raised by QueueByTwoStacks.get() (data_structures/queues/queue_by_two_stacks.py:105). The queue amortizes movement: _stack1 receives put() items and is only flipped into _stack2 when _stack2 is empty. The error fires only when, after the flip attempt, both stacks are empty — i.e. the queue truly holds nothing.

Source

Thrown at data_structures/queues/queue_by_two_stacks.py:105

        1
        >>> queue.get()
        40
        >>> queue.get()
        Traceback (most recent call last):
            ...
        IndexError: Queue is empty
        """

        # To reduce number of attribute look-ups in `while` loop.
        stack1_pop = self._stack1.pop
        stack2_append = self._stack2.append

        if not self._stack2:
            while self._stack1:
                stack2_append(stack1_pop())

        if not self._stack2:
            raise IndexError("Queue is empty")
        return self._stack2.pop()


if __name__ == "__main__":
    from doctest import testmod

    testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Check len(queue) == 0 before calling get() — it counts both internal stacks
2. Use `while queue:` as the drain condition instead of exception-driven looping
3. Catch IndexError if you deliberately detect end-of-data by exception

Example fix

// before
while True:
    process(queue.get())

# after
while queue:
    process(queue.get())

if len(queue) == 0:  # counts both internal stacks
    return None
item = queue.get()

try:
    item = queue.get()
except IndexError as e:
    if str(e) != 'Queue is empty':
        raise
    item = None

When it happens

Trigger: get() on a freshly constructed QueueByTwoStacks(), or one get() beyond the number of put() calls (len(queue) reflects both stacks, so it drops to 0 first).

Common situations: Drain loops that call get() until failure, or producer/consumer code where get() races ahead of put() — note there is no blocking, unlike queue.Queue.get() which waits.

Related errors

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

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/df3b7cc7045f0601. Report an issue: GitHub.