queue
— A synchronized queue class¶
Source code: Lib/queue.py
The queue
module implements multi-producer, multi-consumer queues.
It is especially useful in threaded programming when information must be
exchanged safely between multiple threads. The Queue
class in this
module implements all the required locking semantics. It depends on the
availability of thread support in Python; see the threading
module.
The module implements three types of queue, which differ only in the order in
which the entries are retrieved. In a FIFO
queue, the first tasks added are the first retrieved. In a
LIFO queue, the most recently added entry is
the first retrieved (operating like a stack). With a priority queue,
the entries are kept sorted (using the heapq
module) and the
lowest valued entry is retrieved first.
Internally, those three types of queues use locks to temporarily block competing threads; however, they are not designed to handle reentrancy within a thread.
In addition, the module implements a “simple”
FIFO queue type, SimpleQueue
, whose
specific implementation provides additional guarantees
in exchange for the smaller functionality.
The queue
module defines the following classes and exceptions:
-
class
queue.
Queue
(maxsize=0)¶ Constructor for a FIFO queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.
-
class
queue.
LifoQueue
(maxsize=0)¶ Constructor for a LIFO queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.
-
class
queue.
PriorityQueue
(maxsize=0)¶ Constructor for a priority queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.
The lowest valued entries are retrieved first (the lowest valued entry is the one returned by
sorted(list(entries))[0]
). A typical pattern for entries is a tuple in the form:(priority_number, data)
.If the data elements are not comparable, the data can be wrapped in a class that ignores the data item and only compares the priority number:
from dataclasses import dataclass, field from typing import Any @dataclass(order=True) class PrioritizedItem: priority: int item: Any=field(compare=False)
-
class
queue.
SimpleQueue
¶ Constructor for an unbounded FIFO queue. Simple queues lack advanced functionality such as task tracking.
New in version 3.7.
-
exception
queue.
Empty
¶ Exception raised when non-blocking
get()
(orget_nowait()
) is called on aQueue
object which is empty.
-
exception
queue.
Full
¶ Exception raised when non-blocking
put()
(orput_nowait()
) is called on aQueue
object which is full.
Queue Objects¶
Queue objects (Queue
, LifoQueue
, or PriorityQueue
)
provide the public methods described below.
-
Queue.
qsize
()¶ Return the approximate size of the queue. Note, qsize() > 0 doesn’t guarantee that a subsequent get() will not block, nor will qsize() < maxsize guarantee that put() will not block.
-
Queue.
empty
()¶ Return
True
if the queue is empty,False
otherwise. If empty() returnsTrue
it doesn’t guarantee that a subsequent call to put() will not block. Similarly, if empty() returnsFalse
it doesn’t guarantee that a subsequent call to get() will not block.
-
Queue.
full
()¶ Return
True
if the queue is full,False
otherwise. If full() returnsTrue
it doesn’t guarantee that a subsequent call to get() will not block. Similarly, if full() returnsFalse
it doesn’t guarantee that a subsequent call to put() will not block.
-
Queue.
put
(item, block=True, timeout=None)¶ Put item into the queue. If optional args block is true and timeout is
None
(the default), block if necessary until a free slot is available. If timeout is a positive number, it blocks at most timeout seconds and raises theFull
exception if no free slot was available within that time. Otherwise (block is false), put an item on the queue if a free slot is immediately available, else raise theFull
exception (timeout is ignored in that case).
-
Queue.
put_nowait
(item)¶ Equivalent to
put(item, False)
.
-
Queue.
get
(block=True, timeout=None)¶ Remove and return an item from the queue. If optional args block is true and timeout is
None
(the default), block if necessary until an item is available. If timeout is a positive number, it blocks at most timeout seconds and raises theEmpty
exception if no item was available within that time. Otherwise (block is false), return an item if one is immediately available, else raise theEmpty
exception (timeout is ignored in that case).Prior to 3.0 on POSIX systems, and for all versions on Windows, if block is true and timeout is
None
, this operation goes into an uninterruptible wait on an underlying lock. This means that no exceptions can occur, and in particular a SIGINT will not trigger aKeyboardInterrupt
.
-
Queue.
get_nowait
()¶ Equivalent to
get(False)
.
Two methods are offered to support tracking whether enqueued tasks have been fully processed by daemon consumer threads.
-
Queue.
task_done
()¶ Indicate that a formerly enqueued task is complete. Used by queue consumer threads. For each
get()
used to fetch a task, a subsequent call totask_done()
tells the queue that the processing on the task is complete.If a
join()
is currently blocking, it will resume when all items have been processed (meaning that atask_done()
call was received for every item that had beenput()
into the queue).Raises a
ValueError
if called more times than there were items placed in the queue.
-
Queue.
join
()¶ Blocks until all items in the queue have been gotten and processed.
The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer thread calls
task_done()
to indicate that the item was retrieved and all work on it is complete. When the count of unfinished tasks drops to zero,join()
unblocks.
Example of how to wait for enqueued tasks to be completed:
def worker():
while True:
item = q.get()
if item is None:
break
do_work(item)
q.task_done()
q = queue.Queue()
threads = []
for i in range(num_worker_threads):
t = threading.Thread(target=worker)
t.start()
threads.append(t)
for item in source():
q.put(item)
# block until all tasks are done
q.join()
# stop workers
for i in range(num_worker_threads):
q.put(None)
for t in threads:
t.join()
SimpleQueue Objects¶
SimpleQueue
objects provide the public methods described below.
-
SimpleQueue.
qsize
()¶ Return the approximate size of the queue. Note, qsize() > 0 doesn’t guarantee that a subsequent get() will not block.
-
SimpleQueue.
empty
()¶ Return
True
if the queue is empty,False
otherwise. If empty() returnsFalse
it doesn’t guarantee that a subsequent call to get() will not block.
-
SimpleQueue.
put
(item, block=True, timeout=None)¶ Put item into the queue. The method never blocks and always succeeds (except for potential low-level errors such as failure to allocate memory). The optional args block and timeout are ignored and only provided for compatibility with
Queue.put()
.CPython implementation detail: This method has a C implementation which is reentrant. That is, a
put()
orget()
call can be interrupted by anotherput()
call in the same thread without deadlocking or corrupting internal state inside the queue. This makes it appropriate for use in destructors such as__del__
methods orweakref
callbacks.
-
SimpleQueue.
put_nowait
(item)¶ Equivalent to
put(item)
, provided for compatibility withQueue.put_nowait()
.
-
SimpleQueue.
get
(block=True, timeout=None)¶ Remove and return an item from the queue. If optional args block is true and timeout is
None
(the default), block if necessary until an item is available. If timeout is a positive number, it blocks at most timeout seconds and raises theEmpty
exception if no item was available within that time. Otherwise (block is false), return an item if one is immediately available, else raise theEmpty
exception (timeout is ignored in that case).
-
SimpleQueue.
get_nowait
()¶ Equivalent to
get(False)
.
See also
- Class
multiprocessing.Queue
A queue class for use in a multi-processing (rather than multi-threading) context.
collections.deque
is an alternative implementation of unbounded
queues with fast atomic append()
and
popleft()
operations that do not require locking.