QR helps you create and work with queue, capped collection (bounded queue), deque, and stack data structures for Redis. Redis is well-suited for implementations of these abstract data structures, and QR makes it even easier to work with the structures in Python.
Note: stackd/qr builds upon the work of tnm/qr by serializing data structure elements with cPickle out of the box. This conveniently allows elements to be returned in their native types, as opposed to <'str'>, without extra boilerplate.
You'll need Redis itself -- QR makes use of MULTI/EXEC, so you'll need Redis 2.0 or greater (or one of the later 1.3.x releases). Also necessary is the Python interface for Redis, redis-py. You can install QR with the included setup.py.
Basics of QR
You probably know this already, but here's the 20-second overview of these four data structures.
- You push elements to the back of the queue and pop elements from the front.
- With respect to the elements, it's first in, first out (FIFO).
A capped collection:
- Another name for (what is essentially) a bounded queue.
- You push elements to the back, and once a maximum collection size is reached, the oldest element(s) is trimmed.
A deque, or double-ended queue:
- You can push values to the front or back of a deque, and pop elements from the front or back of the deque.
A stack, or, as they say in German, a 'Stapelspeicher':
- You can push elements to the back of the stack and pop elements from the back of the stack.
- It's last in, first out (LIFO).
Create a QCDS
qr.py includes four little classes: Queue, CappedCollection, Deque, and Stack. To create a new QCDS, just create an instance as follows:
- A first-position key argument is required for all objects. It's the Redis key you want to be associated with the QCDS.
- A second-position size argument is required for CappedCollection. That's how big you want to let the collection get.
Cool, let's create a Beatles queue, circa 1962.
>> from qr import Queue >> bqueue = Queue('Beatles')
You are now the owner of a Queue object (
bqueue), associated with the Redis key 'Beatles'.
>> bqueue.push('Pete') >> bqueue.push('John') >> bqueue.push('Paul') >> bqueue.push('George')
Unfortunately, George Martin doesn't like Pete Best, so it's time to pop him. Since Pete was first in, and this is a queue, after all, we just do this:
>> bqueue.pop() 'Pete'
And, of course, we know who joins the band next.
We can get back (no pun intended) the elements from the queue, too. In fact, each class in QR includes two return-style methods: elements and elements_as_json.
Call elements(), and you'll get back a Python list.
Call elements_as_json(), and you'll get back the list as a JSON object.
>> bqueue.elements() ['Ringo', 'George', 'Paul', 'John'] >> bqueue.elements_as_json() '['Ringo', 'George', 'Paul', 'John']'
A Capped Collection
I don't know if you've heard, but Donald Knuth will be joining Radiohead soon. They need an organ player. Amazing, I know. Anyway, Radiohead has a max of five members, so someone is going to have to get kicked out of the band. Let's demonstrate this with a Capped Collection.
>> from qr import CappedCollection >> radiohead_cc = CappedCollection('Radiohead', 5) >> radiohead_cc.push('Ed') >> radiohead_cc.push('Colin') >> radiohead_cc.push('Thom') >> radiohead_cc.push('Jonny') >> radiohead_cc.push('Phil') >> radiohead_cc.elements() ['Phil', 'Jonny', 'Thom', 'Colin', 'Ed']
Now it's time for Donald to join the group.
And our new Radiohead is :
>> radiohead_cc.elements() ['Donald', 'Phil', 'Jonny', 'Thom', 'Colin']
If you wanted a deque for the Rolling Stones:
>> from qr import Deque >> stones_deque = Deque('Stones')
The deque, of course, has different methods:
The Kinks stack is as easy as:
>> from qr import Stack >> kinks_stack = Stack('Kinks')
The stack has the same methods as the queue.
Feel free to fork!
Thanks to mafr for some initial tests.
Author: Ted Nyman | @tnm
Copyright (c) 2010 Ted Nyman
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