krahets/hello-algo · error · IndexError
队列为空
Error message
队列为空
What it means
IndexError '队列为空' (queue is empty) raised by LinkedListQueue.peek, and inherited by pop which calls peek first. Reading _front.val on an empty queue would dereference None, so the guard intercepts.
Source
Thrown at codes/python/chapter_stack_and_queue/linkedlist_queue.py:56
self._rear = node
# 如果队列不为空，则将该节点添加到尾节点后
else:
self._rear.next = node
self._rear = node
self._size += 1
def pop(self) -> int:
"""出队"""
num = self.peek()
# 删除头节点
self._front = self._front.next
self._size -= 1
return num
def peek(self) -> int:
"""访问队首元素"""
if self.is_empty():
raise IndexError("队列为空")
return self._front.val
def to_list(self) -> list[int]:
"""转化为列表用于打印"""
queue = []
temp = self._front
while temp:
queue.append(temp.val)
temp = temp.next
return queue
"""Driver Code"""
if __name__ == "__main__":
# 初始化队列
queue = LinkedListQueue()
# 元素入队View on GitHub (pinned to 69932aed18)
Solutions
- Check is_empty() before peek or pop.
- Drive with while not q.is_empty(): node = q.pop().
- Return a sentinel for optional reads instead of peeking unconditionally.
Example fix
// before front = q.peek() # raises when empty // after front = q.peek() if not q.is_empty() else None
Defensive patterns
Strategy: validation
Validate before calling
def safe_peek(q):
return q.peek() if not q.is_empty() else None Type guard
def queue_non_empty(q) -> bool:
return not q.is_empty() Try / catch
try:
front = q.peek()
except IndexError:
front = None Prevention
- Guard peek and pop with is_empty().
- Use while not q.is_empty() in BFS consumers.
- Start consumers only after the first push.
When it happens
Trigger: Calling queue.peek() or queue.pop() when _size == 0.
Common situations: BFS where the frontier is popped after exhaustion; level-order traversal whose termination check is off by one; consumer that starts before the producer's first push.
Related errors
AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13).
Data as JSON: /api/errors/d0b042392982d4ef.
Report an issue: GitHub.