python : how to know max size of a queue

Question

Size of a queue can be known by using qsize()

• How do we know the max size of a queue? Or in other way, when the condition queue.full() will return True?

• Why is the result for q.put(i) always resulting in None whereas q.get(i) is resulting in corresponding value?

  from queue import Queue
  q = Queue()
  
  print("------Queue info when putting -----")
  
  for i in range(5):
     print("---Loop---- :",i) 
     print("queue empty : ", q.empty())
     print("put : ",q.put(i))
     print("queue size : ", q.qsize())
     print("queue full : ", q.full())
  
  print("------Queue info when getting -----")        
  
  for i in range(5): 
     print("---Loop---- :",i)
     print("get : ",q.get(i))
     print("task done : ",q.task_done())
     print("queue empty : ", q.empty())
  

---

output is as follows :

------Queue info when putting -----     
---Loop---- : 0    
queue empty :  True    
put :  None    
queue size :  1    
queue full :  False    
(loop continues for 5 times)


------Queue info when getting -----
---Loop---- : 0    
get :  0    
task done :  None    
queue empty :  False     
(loop continues for 5 times)

Solution 1

To answer your 1st question: for all intents and purposes, the max size of a Queue is infinite. The reason why is that if you try to put something in a Queue that is full, it will wait until a slot has opened up before it puts the next item into the queue. This is assuming you initialize the Queue with the max size parameter. You can see here: class queue.Queue(maxsize) that you can pass a maxsize parameter. According to the docs: " If maxsize is less than or equal to zero, the queue size is infinite."

This leads to the second question: the put() method puts the item into the queue and returns None if it was successful. From the documentation,

put(item[, block[, timeout]])

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).

Therefore, when you put something into the queue, it will always return None unless an exception was raised. get(), on the other hand, returns the item it gets. From the docs again:

get([block[, timeout]]): Remove and return an item from the queue.

Solution 2

Check out the source code for the implementation of Queue: https://hg.python.org/cpython/file/3.5/Lib/queue.py

As you can see from the init function, 'queue.maxsize' is a member variable that should contain the max size of the queue.

You can therefore do something like this:

from queue import Queue

my_queue = Queue(maxsize=10)
max_size = queue.maxsize

Comments

[Adrian]

The reason why is that if you try to put something in a Queue that is full, it will wait until a slot has opened up before it puts the next item into the queue. What about put(item, False) or the equivalent put_nowait(item)? It won't wait until an item is consumed, it will just raise Full. So, no, it's not infinite for all intents and purposes.