Installation

You can install Queuelib either via the Python Package Index (PyPI) or from source.

To install using pip:

$ pip install queuelib

To install using easy_install:

$ easy_install queuelib

If you have downloaded a source tarball you can install it by running the following (as root):

# python setup.py install

FIFO/LIFO disk queues

Queuelib provides FIFO and LIFO queue implementations.

Here is an example usage of the FIFO queue:

>>> from queuelib import FifoDiskQueue
>>> q = FifoDiskQueue("queuefile")
>>> q.push(b'a')
>>> q.push(b'b')
>>> q.push(b'c')
>>> q.pop()
b'a'
>>> q.close()
>>> q = FifoDiskQueue("queuefile")
>>> q.pop()
b'b'
>>> q.pop()
b'c'
>>> q.pop()
>>>

The LIFO queue is identical (API-wise), but importing LifoDiskQueue instead.

PriorityQueue

A discrete-priority queue implemented by combining multiple FIFO/LIFO queues (one per priority).

First, select the type of queue to be used per priority (FIFO or LIFO):

>>> from queuelib import FifoDiskQueue
>>> qfactory = lambda priority: FifoDiskQueue('queue-dir-%s' % priority)

Then instantiate the Priority Queue with it:

>>> from queuelib import PriorityQueue
>>> pq = PriorityQueue(qfactory)

And use it:

>>> pq.push(b'a', 3)
>>> pq.push(b'b', 1)
>>> pq.push(b'c', 2)
>>> pq.push(b'd', 2)
>>> pq.pop()
b'b'
>>> pq.pop()
b'c'
>>> pq.pop()
b'd'
>>> pq.pop()
b'a'

RoundRobinQueue

Has nearly the same interface and implementation as a Priority Queue except that each element must be pushed with a (mandatory) key. Popping from the queue cycles through the keys “round robin”.

Instantiate the Round Robin Queue similarly to the Priority Queue:

>>> from queuelib import RoundRobinQueue
>>> rr = RoundRobinQueue(qfactory)

And use it:

>>> rr.push(b'a', '1')
>>> rr.push(b'b', '1')
>>> rr.push(b'c', '2')
>>> rr.push(b'd', '2')
>>> rr.pop()
b'a'
>>> rr.pop()
b'c'
>>> rr.pop()
b'b'
>>> rr.pop()
b'd'

Clearing a queue

clear() removes every item from a queue, freeing the disk space that they used, and leaves the queue open and usable:

>>> q.clear()
>>> len(q)
0

PriorityQueue and RoundRobinQueue also close their internal queues, the same way that pop() does when one of them becomes empty.

Disk persistence

FifoDiskQueue and LifoDiskQueue write their items to the path they get on instantiation, so that a queue can be resumed later, even by a different process.

Each class uses that path differently:

• FifoDiskQueue uses a directory, which it creates, together with any missing parent directory. Items go into chunk files (q00000, q00001, etc.), each holding up to chunksize items, and the queue also keeps an info.json file there for its own bookkeeping.

• LifoDiskQueue uses a single file, whose parent directory must already exist.

The layout and the contents of those files are an implementation detail that may change in any release. Do not read or write them yourself, and do not expect a queue written by one version of Queuelib to be readable by a different one.

from contextlib import closing

with closing(FifoDiskQueue("queuedir")) as q:
    q.push(b'a')

>>> import json
>>> from queuelib import FifoDiskQueue, PriorityQueue
>>> qfactory = lambda priority: FifoDiskQueue('queue-dir-%s' % priority)
>>> pq = PriorityQueue(qfactory)
>>> pq.push(b'a', 3)
>>> active = pq.close()
>>> with open('active.json', 'w') as f:
...     json.dump(active, f)
...
>>> with open('active.json') as f:
...     startprios = json.load(f)
...
>>> pq = PriorityQueue(qfactory, startprios)
>>> pq.pop()
b'a'

Bug tracker

If you have any suggestions, bug reports or annoyances please report them to our issue tracker at: http://github.com/scrapy/queuelib/issues/

Contributing

Development of Queuelib happens at GitHub: http://github.com/scrapy/queuelib

You are highly encouraged to participate in the development. If you don’t like GitHub (for some reason) you’re welcome to send regular patches.

All changes require tests to be merged.

Tests

Tests are located in queuelib/tests directory. They can be run using nosetests with the following command:

nosetests

The output should be something like the following:

$ nosetests
.............................................................................
----------------------------------------------------------------------
Ran 77 tests in 0.145s

OK

License

This software is licensed under the BSD License. See the LICENSE file in the top distribution directory for the full license text.

Versioning

This software follows Semantic Versioning