Queue

Learn inter-process queue with multiprocessing.Queue.

What is multiprocessing.Queue?

multiprocessing.Queue is a thread-safe FIFO queue for passing data between processes. You put data with .put() and get it with .get().

Multiple processes can write to and read from the same queue; internal locking guarantees ordering. This is the most common way to send a process's result back to the main program. For example a worker process does q.put(result) and the main program receives it with q.get(). If the queue is empty get waits until data arrives.

Passing data with Queue

A Queue can be used by both the main program and a worker process. The worker puts a result with q.put(result) and the main program gets it with q.get().

The key point: the Queue is passed to the process as an argument. You pass it by writing Process(target=worker, args=(q,)). The worker writes to this queue and the main program reads from it. The queue is FIFO so the first item put is the first taken. If multiple workers write to the same queue the order depends on write speed; but each item is taken only once.

Exercise Pass values with Queue

Import with from multiprocessing import Queue. Create a queue with q = Queue(). Put two values with q.put("bir") and q.put("iki"). Get the two values in order with q.get() and print them.

Show example solution

from multiprocessing import Queue

q = Queue()
q.put("bir")
q.put("iki")
print(q.get())
print(q.get())

Test yourself

Complete the Queue call

from multiprocessing import ____
q = Queue()

Show answer

from multiprocessing import Queue
q = Queue()

What does q.get() do?
```
q = Queue()
q.put(42)
val = q.get()
```
- Puts a value into the queue
- Takes a value from the queue
- Deletes the queue
- Returns the queue size
Show answer

Takes a value from the queue — get() takes a value from the front of the queue.
Match Queue concepts

put

Puts a value
get

Takes a value
Queue

FIFO queue
args=(q,)

Passes the queue

Next lesson: Pool

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