Queuelib is a Python library that implements object collections which are stored in memory or persisted to disk, provide a simple API, and run fast.
Queuelib provides collections for queues (FIFO), stacks (LIFO), queues sorted by priority and queues that are emptied in a round-robin fashion.
Note
Queuelib collections are not thread-safe.
Queuelib supports Python 3.10+ and has no dependencies.
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
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.
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'
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'
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:
FifoDiskQueueuses a directory, which it creates, together with any missing parent directory. Items go into chunk files (q00000,q00001, etc.), each holding up tochunksizeitems, and the queue also keeps aninfo.jsonfile there for its own bookkeeping.LifoDiskQueueuses 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.
While a disk queue is open, its bookkeeping (number of items, read and write
positions) only lives in memory, and close() is what writes it to disk.
Queuelib never calls fsync() either, and LifoDiskQueue writes items
through a buffered file object, so the most recent items may not have reached
the disk at all.
Calling close() is hence mandatory:
from contextlib import closing
with closing(FifoDiskQueue("queuedir")) as q:
q.push(b'a')
If a process ends without calling close(), the queue on disk keeps the
bookkeeping that the last close() call wrote, which no longer matches the
files. Items pushed since then become unreachable, and using the queue again
is unsafe: it may report a wrong length, return items that had already been
popped, delete files that still contain items, or raise OSError. Queuelib
offers no way to repair or to recover such a queue.
close() on an empty queue deletes its file, or, in the case of
FifoDiskQueue, its chunk files and its info.json file, and also its
directory if nothing else remains in it. Using that same path again creates a
new, empty queue.
FifoDiskQueue stores its chunksize when creating a queue, and reuses
the stored value when reopening one, ignoring the chunksize parameter.
Queuelib does not lock the files that it uses. On top of not being
thread-safe, a given path must not be used by more than one open queue object
at a time, in the same process or not. Such queue objects overwrite each
other's items and bookkeeping; for example, two FifoDiskQueue objects on
the same directory return the same items, and their close() calls may
raise FileNotFoundError.
PriorityQueue and RoundRobinQueue do not write anything to disk
themselves; their persistence comes entirely from the queues that qfactory
builds, and it is up to qfactory to map a priority or a key to a valid
path.
Their close() method returns the priorities or keys whose underlying queue
was not empty. Storing that value is your responsibility, and so is passing it
back as startprios or start_domains on the next run:
>>> 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'
Priorities and keys that you do not pass back are not detected, and the items in their queues stay on disk, unreachable.
If you have any suggestions, bug reports or annoyances please report them to our issue tracker at: http://github.com/scrapy/queuelib/issues/
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 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
This software is licensed under the BSD License. See the LICENSE file in the top distribution directory for the full license text.
This software follows Semantic Versioning