krahets/hello-algo · error · IndexError
佇列為空
Error message
佇列為空
What it means
Raised by the peek() method of a singly-linked-list-based queue when the internal _front pointer is None (i.e., _size == 0). Since pop() calls peek() to read the front value before advancing _front = _front.next, both operations fail on an empty queue. Without the guard, pop() would dereference None.next and crash with AttributeError.
Source
Thrown at zh-hant/codes/python/chapter_stack_and_queue/linkedlist_queue.py:56
self._rear = node
# 如果佇列不為空，則將該節點新增到尾節點後
else:
self._rear.next = node
self._rear = node
self._size += 1
def pop(self) -> int:
"""出列"""
num = self.peek()
# 刪除頭節點
self._front = self._front.next
self._size -= 1
return num
def peek(self) -> int:
"""訪問佇列首元素"""
if self.is_empty():
raise IndexError("佇列為空")
return self._front.val
def to_list(self) -> list[int]:
"""轉化為串列用於列印"""
queue = []
temp = self._front
while temp:
queue.append(temp.val)
temp = temp.next
return queue
"""Driver Code"""
if __name__ == "__main__":
# 初始化佇列
queue = LinkedListQueue()
# 元素入列View on GitHub (pinned to 69932aed18)
Solutions
- Check queue.is_empty() before calling peek() or pop()
- Use while not queue.is_empty() as the draining loop condition
- Wrap in try/except IndexError for defensive queue operations
Example fix
# before
val = queue.pop()
# after
if not queue.is_empty():
val = queue.pop()
else:
val = None Defensive patterns
Strategy: validation
Validate before calling
if not queue.is_empty():
val = queue.pop()
else:
val = None Try / catch
try:
val = queue.pop()
except IndexError:
val = None Prevention
- Check is_empty() before peek() or pop() in consumer loops
- pop() delegates to peek(), so both share the same guard
- Use while not queue.is_empty() as the BFS draining condition
When it happens
Trigger: Calling peek() or pop() on a freshly constructed LinkedListQueue; draining all elements and then calling pop() once more; a consumer processing items faster than the producer enqueues them.
Common situations: BFS traversal that processes all nodes and then attempts one more dequeue; task queues that are momentarily empty between producer batches; testing with empty input collections.
Related errors
AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13).
Data as JSON: /api/errors/ba940a93db851ed2.
Report an issue: GitHub.