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.
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 that would be returned by min(entries)
). 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()
(or
get_nowait()
) is called
on a Queue
object which is empty.
exception
queue.
Full
¶Exception raised when non-blocking put()
(or
put_nowait()
) is called
on a Queue
object which is full.
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()
returns True
it doesn’t guarantee that a subsequent call to put()
will not block. Similarly, if empty() returns False
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()
returns True
it doesn’t guarantee that a subsequent call to get()
will not block. Similarly, if full() returns False
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
the Full
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 the Full
exception (timeout is
ignored in that case).
Queue.
put_nowait
(
item
)
¶Equivalent to put(item, block=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 the Empty
exception if no item was available within that time.
Otherwise (block is false), return an item if one is immediately available,
else raise the Empty
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 a KeyboardInterrupt
.
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 to
task_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 a task_done()
call was received for every item
that had been put()
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:
import
threading
import
queue
q
=
queue
.
Queue
()
def
worker
():
while
True
:
item
=
q
.
get
()
(
f
'Working on
{
item
}
'
)
(
f
'Finished
{
item
}
'
)
q
.
task_done
()
# Turn-on the worker thread.
threading
.
Thread
(
target
=
worker
,
daemon
=
True
)
.
start
()
# Send thirty task requests to the worker.
for
item
in
range
(
30
):
q
.
put
(
item
)
# Block until all tasks are done.
q
.
join
()
(
'All work completed'
)
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()
returns False
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()
or get()
call can be interrupted by another put()
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 or weakref
callbacks.
SimpleQueue.
put_nowait
(
item
)
¶Equivalent to put(item, block=False)
, provided for compatibility with
Queue.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 the Empty
exception if no item was available within that time.
Otherwise (block is false), return an item if one is immediately available,
else raise the Empty
exception (timeout is ignored in that case).
SimpleQueue.
get_nowait
(
)
¶Equivalent to get(False)
.
See also
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
and also support indexing.
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