mission-peace/interview · warning · KeyError
pop from an empty priority queue
Error message
pop from an empty priority queue
What it means
pop_task drains stale (removed-flagged) heap entries; if the heap ends up empty with no live task it raises KeyError("pop from an empty priority queue"). This signals a dequeue attempt when no tasks remain.
Solutions
- Check pq.is_empty() before popping
- Catch KeyError around pop_task in consumer loops and treat as 'queue empty'
- Use a sentinel/blocking mechanism for wait-until-available semantics
Example fix
// before
task = pq.pop_task()
// after
try:
task = pq.pop_task()
except KeyError:
task = None Defensive patterns
Strategy: try-catch
Validate before calling
if pq.entry_finder:
task = pq.pop_task() Try / catch
try:
task = pq.pop_task()
except KeyError:
task = None # queue empty Prevention
- Guard consumer loops with is_empty() or KeyError catch
- Synchronize producers/consumers if multiple threads drain the queue
- Never assume a task is pending without checking entry_finder
When it happens
Trigger: Calling pop_task() on a queue with no pending tasks, or where all entries were invalidated via remove_task.
Common situations: Worker loops that poll without checking emptiness, race where another consumer drained the queue, or draining after processing completed.
Understand the failure class
Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.
Related errors
AI-assisted analysis of mission-peace/interview@94be5deb0c (2026-09-08).
Data as JSON: /api/errors/56f329d791018584.
Report an issue: GitHub.
Appendix: source
Thrown at python/graph/priorityqueue.py:53
def change_task_priority(self, priority, task):
if task not in self.entry_finder:
raise KeyError("Task not found")
self.remove_task(task)
entry = [self.mul*priority, False, task]
self.entry_finder[task] = entry
heappush(self.pq, entry)
def remove_task(self, task):
entry = self.entry_finder.pop(task)
entry[1] = True
def pop_task(self):
while self.pq:
priority, removed, task = heappop(self.pq)
if removed is False:
del self.entry_finder[task]
return task
raise KeyError("pop from an empty priority queue")
def peek_task(self):
while self.pq:
priority, removed, task = tuple(heappop(self.pq))
if removed is False:
heappush(self.pq, [priority, False, task])
return task
raise KeyError("pop from an empty priority queue")
def is_empty(self):
try:
self.peek_task()
return False
except KeyError:
return True
def __str__(self):
return str(self.entry_finder) + " " + str(self.pq)View on GitHub (pinned to 94be5deb0c)