Author Dusty Phillips Reviewers

LIFO
(
Last In First Out
) queues are more frequently called 
stacks
. Think of a stack 
of papers where you can only access the top-most paper. You can put another paper 
on top of the stack, making it the new top-most paper, or you can take the top-most 
paper away to reveal the one beneath it.
Traditionally, the operations on stacks are named push and pop, but the Python
queue
module uses the exact same API as for FIFO queues: 
put()
and 
get()
. 
However, in a LIFO queue, these methods operate on the "top" of the stack instead
of at the front and back of a line. This is an excellent example of polymorphism. If you 
look at the 
Queue
source code in the Python standard library, you'll actually see that 
there is a superclass with subclasses for FIFO and LIFO queues that implement the 
few operations (operating on the top of a stack instead of front and back of a 
deque
instance) that are critically different between the two.
Here's an example of the LIFO queue in action:
>>> from queue import LifoQueue
>>> stack = LifoQueue(maxsize=3)
>>> stack.put("one")
>>> stack.put("two")
>>> stack.put("three")
>>> stack.put("four", block=False)
Traceback (most recent call last):
File "", line 1, in
stack.put("four", block=False)
File "/usr/lib64/python3.3/queue.py", line 133, in put
raise Full
queue.Full
>>> stack.get()
'three'
>>> stack.get()
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Python Data Structures
[
 186 
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'two'
>>> stack.get()