Priority Queues
A priority queue is a container data structure that manages a set of records with totally-ordered keys (for example, a numeric weight value) to provide quick access to the record with the smallest or largest key in the set.
You can think of a priority queue as a modified queue: instead of retrieving the next element by insertion time, it retrieves the highestpriority element. The priority of individual elements is decided by the ordering applied to their keys.
你可以认为优先队列是修改的队列。它修改的是最高优先级的元素。单个元素的优先级由它们对应的键的顺序决定。
Priority queues are commonly used for dealing with scheduling problems, for example, to give precedence to tasks with higher urgency.
Think about the job of an operating system task scheduler:
Ideally, high-priority tasks on the system (e.g., playing a real-time game) should take precedence over lower-priority tasks (e.g., downloading updates in the background). By organizing pending tasks in a priority queue that uses the task urgency as the key, the task scheduler can quickly select the highest-priority tasks and allow them to run first.
In this chapter you’ll see a few options for how you can implement Priority Queues in Python using built-in data structures or data structures that ship with Python’s standard library. Each implementation will have their own upsides and downsides, but in my mind there’s a clear winner for most common scenarios. Let’s find out which one it is.
list – Maintaining a Manually Sorted Queue
You can use a sorted list to quickly identify and delete the smallest or largest element. The downside is that inserting new elements into a list is a slow O(n) operation.
While the insertion point can be found in O(log n) time using bisect.insort in the standard library, this is always dominated by the slow insertion step.
Maintaining the order by appending to the list and re-sorting also takes at least O(n log n) time. Another downside is that you must manually take care of re-sorting the list when new elements are inserted. It’s easy to introduce bugs by missing this step, and the burden is always on you, the developer.
Therefore, I believe that sorted lists are only suitable as priority queues when there will be few insertions.
在插入少的时候排序列表才是适合的。
q = []
q.append((2, 'code'))
q.append((1, 'eat'))
q.append((3, 'sleep'))
# NOTE: Remember to re-sort every time
# a new element is inserted, or use
# bisect.insort().
q.sort(reverse=True)
while q:
next_item = q.pop()
print(next_item)
# Result:
# (1, 'eat')
# (2, 'code')
# (3, 'sleep')
heapq – List-Based Binary Heaps
This is a binary heap implementation usually backed by a plain list, and it supports insertion and extraction of the smallest element in O(log n) time.
This module is a good choice for implementing priority queues in Python. Since heapq technically only provides a min-heap implementation, extra steps must be taken to ensure sort stability and other features typically expected from a “practical” priority queue.
import heapq
q = []
heapq.heappush(q, (2, 'code'))
heapq.heappush(q, (1, 'eat'))
heapq.heappush(q, (3, 'sleep'))
while q:
next_item = heapq.heappop(q)
print(next_item)
# Result:
# (1, 'eat')
# (2, 'code')
# (3, 'sleep')
queue.PriorityQueue – Beautiful Priority Queues
This priority queue implementation uses heapq internally and shares the same time and space complexities.
The difference is that PriorityQueue is synchronized and provides locking semantics to support multiple concurrent producers and consumers.
Depending on your use case, this might be helpful—or just slow your program down slightly. In any case, you might prefer the class-based interface provided by PriorityQueue over using the function-based interface provided by heapq.
from queue import PriorityQueue
q = PriorityQueue()
q.put((2, 'code'))
q.put((1, 'eat'))
q.put((3, 'sleep'))
while not q.empty():
next_item = q.get()
print(next_item)
# Result:
# (1, 'eat')
# (2, 'code')
# (3, 'sleep')
从上面的操作过程来看我们也可以知道heapq是用函数包装的，而priorityqueue是基于类搭建起来的。
Key Takeaways
- Python includes several priority queue implementations for you to use.
- queue.PriorityQueue stands out from the pack with a nice object-oriented interface and a name that clearly states its intent. It should be your preferred choice.
- If you’d like to avoid the locking overhead of queue.PriorityQueue, using the heapq module directly is also a good option.
