Add lossy queue to queue library module

[bpb]

BPO	7337
Nosy	@rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2010-04-03.03:17:50.785>
created_at = <Date 2009-11-17.10:40:39.579>
labels = ['type-feature', 'library']
title = 'Add lossy queue to queue library module'
updated_at = <Date 2010-04-03.03:17:50.783>
user = 'https://bugs.python.org/bpb'

bugs.python.org fields:

activity = <Date 2010-04-03.03:17:50.783>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2010-04-03.03:17:50.785>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2009-11-17.10:40:39.579>
creator = 'bpb'
dependencies = []
files = []
hgrepos = []
issue_num = 7337
keywords = []
message_count = 4.0
messages = ['95374', '95394', '95415', '102217']
nosy_count = 2.0
nosy_names = ['rhettinger', 'bpb']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue7337'
versions = ['Python 3.2']

[bpb]

Many applications would benefit from 'connectionless' queues, i.e. they
don't want to care whether anything is reading from the other end.
Using current queue module classes this is not practical, because there
is a choice between unbounded memory consumption or blocking. I propose
adding a 'LossyQueue' class in the queue module which would allow
bounded memory consumption without blocking on put. (i.e. items are
dropped in fifo manner beyond a certain limit). In my view this is at
least as natural as the PriorityQueue and LifoQueue extensions in that
module.

Outline as follows:

class LossyQueue(Queue):
    "Queue subclass which drops items on overflow"
    def _init(self, maxsize):
        if maxsize > 0:
            # build the deque with maxsize limit
            self.queue = deque(maxlen=maxsize)
        else:
            # same as normal Queue instance
            self.queue = collections.deque()
        # deque alone handles maxsize,
        # so we pretend we have none
        self.maxsize = 0

if there is interest in this I will offer a proper patch with docs and
tests.

[rhettinger]

I'm curious about your "many applications would benefit from
'connectionless' queues". What do you have in mind? Is there any
reason those apps cannot use collections.deque() directly?

[bpb]

'connectionless' is from how I see it as an analogy with UDP (vs TCP);
why not just use a deque is primarily about having the same API - a
client (getter) of the queue shouldn't know or care whether it is a
'lossy' queue or a normal queue. I guess most uses of a normal queue
(excepting the 'task' functions) could just use a deque, but it wouldn't
feel natural.

Use cases: non-critical event/status reporting is my canonical example.
Specific examples:

• a program which executes a long running process in a thread. It wants
  to update a GUI progress bar or similar, which must occur in a different
  thread because of the GUI model. By using a LossyQueue, the server
  thread is simplified; it doesn't have to care whether anything is
  listening on the other end, allowing greater decoupling (e.g. no changes
  required if there isn't a GUI). LossyQueues become part of the interface
  which can be used or not as required.
• emulating/providing wrapper around UDP sockets
• many application protocols support a get/set/report type interface
  with the addition of asynchronous events (e.g. SNMP, Netconf, SCPI). In
  these type of applications a suitable abstraction might be a normal
  Queue(s) for the standard commands and a LossyQueue for the events
  (which some applications might not care about). The point is that to
  the user of this abstraction, these two interfaces look the same.

The 'server' doesn't care if a client is listening or not (it won't
block and it won't use unlimited memory)
The 'client' (if it wants to use it) doesn't know that it isn't a normal
queue (same API).
-> decouples server and client tasks.

[rhettinger]

I think it would be better to put this in the ASPN recipes cookbook to let it mature and gather a following. Right now, it is not at all clear that this is the right thing to do.