queue.py

#
# Licensed to the Apache Software Foundation (ASF) under one
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# distributed with this work for additional information
# regarding copyright ownership.  The ASF licenses this file
# to you under the Apache License, Version 2.0 (the
# "License"); you may not use this file except in compliance
# with the License.  You may obtain a copy of the License at
# 
#   http://www.apache.org/licenses/LICENSE-2.0
# 
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
# KIND, either express or implied.  See the License for the
# specific language governing permissions and limitations
# under the License.
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"""
This module augments the standard python multithreaded Queue
implementation to add a close() method so that threads blocking on the
content of a queue can be notified if the queue is no longer in use.
"""

from __future__ import absolute_import
try:
  from Queue import Queue as BaseQueue, Empty, Full
except ImportError:
  from queue import Queue as BaseQueue, Empty, Full
from threading import Thread
from .exceptions import Closed

class Queue(BaseQueue):

  END = object()
  STOP = object()

  def __init__(self, *args, **kwargs):
    BaseQueue.__init__(self, *args, **kwargs)
    self.error = None
    self.listener = None
    self.exc_listener = None
    self.thread = None

  def close(self, error = None):
    if error and self.error is None:
      self.error = error
    self.put(Queue.END)
    if self.thread is not None:
      self.thread.join()
      self.thread = None

  def get(self, block = True, timeout = None):
    result = BaseQueue.get(self, block, timeout)
    if result == Queue.END:
      # this guarantees that any other waiting threads or any future
      # calls to get will also result in a Closed exception
      self.put(Queue.END)
      raise Closed(self.error)
    else:
      return result

  def listen(self, listener, exc_listener = None):
    if listener is None and exc_listener is not None:
      raise ValueError("cannot set exception listener without setting listener")

    if listener is None:
      if self.thread is not None:
        self.put(Queue.STOP)
        # loop and timed join permit keyboard interrupts to work
        while self.thread.is_alive():
          self.thread.join(3)
        self.thread = None

    self.listener = listener
    self.exc_listener = exc_listener

    if listener is not None and self.thread is None:
      self.thread = Thread(target = self.run)
      self.thread.daemon = True
      self.thread.start()

  def run(self):
    while True:
      try:
        o = self.get()
        if o == Queue.STOP: break
        self.listener(o)
      except Closed as e:
        if self.exc_listener is not None:
          self.exc_listener(e)
        break