Author Dusty Phillips Reviewers

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A priority queue might be used, for example, by a search engine to ensure it refreshes 
the content of the most popular web pages before crawling sites that are less likely to 
be searched for. A product recommendation tool might use one to display information 
about the most highly ranked products while still loading data for the lower ranks.
Note that a priority queue will always return the most important element currently 
in the queue. The 
get()
method will block (by default) if the queue is empty, but it 
will not block and wait for a higher priority element to be added if there is already 
something in the queue. The queue knows nothing about elements that have not 
been added yet (or even about elements that have been previously extracted), and 
only makes decisions based on the current contents of the queue.
This interactive session shows a priority queue in action, using tuples as weights to 
determine what order items are processed in:
>>> heap.put((3, "three"))
>>> heap.put((4, "four"))
>>> heap.put((1, "one") )
>>> heap.put((2, "two"))
>>> heap.put((5, "five"), block=False)
Traceback (most recent call last):
File "", line 1, in
heap.put((5, "five"), block=False)
File "/usr/lib64/python3.3/queue.py", line 133, in put
raise Full
Full
>>> while not heap.empty():
print(heap.get())
(1, 'one')
(2, 'two')
(3, 'three')
(4, 'four')
Priority queues are almost universally implemented using the 
heap
data structure. 
Python's implementation utilizes the 
heapq
module to effectively store a heap inside 
a normal list. I direct you to an algorithm and data-structure's textbook for more 
information on heaps, not to mention many other fascinating structures we haven't 
covered here. No matter what the data structure, you can use object-oriented principles 
to wrap relevant algorithms (behaviors), such as those supplied in the 
heapq
module, 
around the data they are structuring in the computer's memory, just as the 
queue
module has done on our behalf in the standard library.
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Python Data Structures