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# coding: utf-8
:copyright 2010-2011 by Daniel Neuhäuser
:license: BSD, see LICENSE.rst for details
import Queue as queue
class SetQueue(queue.Queue):
"""Thread-safe implementation of an ordered set queue, which coalesces
duplicate items into a single item if the older occurrence has not yet been
read and maintains the order of items in the queue.
Ordered set queues are useful when implementing data structures like
event buses or event queues where duplicate events need to be coalesced
into a single event. An example use case is the inotify API in the Linux
kernel which shares the same behaviour.
Queued items must be immutable and hashable so that they can be used as
dictionary keys or added to sets. Items must have only read-only properties
and must implement the :meth:`__hash__`, :meth:`__eq__`, and :meth:`__ne__`
methods to be hashable.
An example item class implementation follows::
class QueuedItem(object):
def __init__(self, a, b):
self._a = a
self._b = b
def a(self):
return self._a
def b(self):
return self._b
def _key(self):
return (self._a, self._b)
def __eq__(self, item):
return self._key() == item._key()
def __ne__(self, item):
return self._key() != item._key()
def __hash__(self):
return hash(self._key())
.. NOTE::
This ordered set queue leverages locking already present in the
:class:`queue.Queue` class redefining only internal primitives. The
order of items is maintained because the internal queue is not replaced.
An internal set is used merely to check for the existence of an item in
the queue.
.. versionadded:: 0.3
:author: Yesudeep Mangalapilly <>
:author: Lukáš Lalinský <>
def _init(self, maxsize):
queue.Queue._init(self, maxsize)
self._set_of_items = set()
def _put(self, item):
if item not in self._set_of_items:
queue.Queue._put(self, item)
def _get(self):
item = queue.Queue._get(self)
return item
__all__ = ['SetQueue']
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