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|
"""Contains a queue based channel implementation"""
from Queue import (
Empty,
Full
)
from util import (
AsyncQueue,
ReadOnly
)
from time import time
import sys
#{ Classes
class Channel(object):
"""A channel is similar to a file like object. It has a write end as well as one or
more read ends. If Data is in the channel, it can be read, if not the read operation
will block until data becomes available.
If the channel is closed, any read operation will result in an exception
This base class is not instantiated directly, but instead serves as constructor
for RWChannel pairs.
Create a new channel """
__slots__ = tuple()
def __new__(cls, *args):
if cls is Channel:
if len(args) > 0:
raise ValueError("Cannot take any arguments when creating a new channel")
wc = WChannel()
rc = RChannel(wc)
return wc, rc
# END constructor mode
return object.__new__(cls)
class WChannel(Channel):
"""The write end of a channel"""
__slots__ = ('_closed', '_queue')
def __init__(self):
"""initialize this instance, able to hold max_items at once
Write calls will block if the channel is full, until someone reads from it"""
self._closed = False
self._queue = AsyncQueue()
#{ Interface
def write(self, item, block=True, timeout=None):
"""Send an item into the channel, it can be read from the read end of the
channel accordingly
:param item: Item to send
:param block: If True, the call will block until there is free space in the
channel
:param timeout: timeout in seconds for blocking calls.
:raise IOError: when writing into closed file
:raise EOFError: when writing into a non-blocking full channel"""
# let the queue handle the 'closed' attribute, we write much more often
# to an open channel than to a closed one, saving a few cycles
try:
self._queue.put(item, block, timeout)
except ReadOnly:
raise IOError("Cannot write to a closed channel")
# END exception handling
def size(self):
""":return: approximate number of items that could be read from the read-ends
of this channel"""
return self._queue.qsize()
def close(self):
"""Close the channel. Multiple close calls on a closed channel are no
an error"""
# yes, close it a little too early, better than having anyone put
# additional items
# print "closing channel", self
self._closed = True
self._queue.set_writable(False)
@property
def closed(self):
""":return: True if the channel was closed"""
return self._closed
#} END interface
class RChannel(Channel):
"""The read-end of a corresponding write channel"""
__slots__ = '_wc'
def __init__(self, wchannel):
"""Initialize this instance from its parent write channel"""
self._wc = wchannel
#{ Interface
def read(self, count=0, block=True, timeout=None):
"""read a list of items read from the channel. The list, as a sequence
of items, is similar to the string of characters returned when reading from
file like objects.
:param count: given amount of items to read. If < 1, all items will be read
:param block: if True, the call will block until an item is available
:param timeout: if positive and block is True, it will block only for the
given amount of seconds, returning the items it received so far.
:return: single item in a list if count is 1, or a list of count items.
If the channel was empty and count was 1, an empty list will be returned.
If count was greater 1, a list with less than count items will be
returned.
If count was < 1, a list with all items that could be read will be
returned."""
# if the channel is closed for writing, we never block
# NOTE: is handled by the queue
if self._wc.closed or timeout == 0:
block = False
# in non-blocking mode, its all not a problem
out = list()
queue = self._wc._queue
if not block:
# be as fast as possible in non-blocking mode, hence
# its a bit 'unrolled'
try:
if count == 1:
out.append(queue.get(False))
elif count < 1:
while True:
out.append(queue.get(False))
# END for each item
else:
for i in xrange(count):
out.append(queue.get(False))
# END for each item
# END handle count
except Empty:
pass
# END handle exceptions
else:
# to get everything into one loop, we set the count accordingly
if count == 0:
count = sys.maxint
# END handle count
endtime = sys.maxint # allows timeout for whole operation
if timeout is not None:
endtime = time() + timeout
# could be improved by a separate: no-endtime branch, saving the time calls
for i in xrange(count):
try:
out.append(queue.get(block, timeout))
except Empty:
# here we are only if there is nothing on the queue,
# and if we are blocking. If we are not blocking, this
# indiccates that the queue was set unwritable in the meanwhile.
# hence we can abort now to prevent reading (possibly) forever
# Besides, this is racy as all threads will rip on the channel
# without waiting until its empty
if not block:
break
# END ignore empty
# if we have been unblocked because the closed state changed
# in the meanwhile, stop trying
# NOTE: must NOT cache _wc
if self._wc.closed:
# If we were closed, we drop out even if there might still
# be items. Now its time to get these items, according to
# our count. Just switch to unblocking mode.
# If we are to read unlimited items, this would run forever,
# but the EmptyException handler takes care of this
block = False
# we don't continue, but let the timer decide whether
# it wants to abort
# END handle channel cloased
if time() >= endtime:
break
# END stop operation on timeout
# END for each item
# END handle blocking
return out
#} END interface
#} END classes
|
