Stack and Queue in Python using queue Module

In Python, it is very easy to implement stack and queue data structures. Stack is called LIFO because Stack works on the principle of "Last-in, first-out" and Queue is called FIFO because Queue works on the principle of "First-in, first-out", and the inbuilt functions in Python make the code shorter and simple.

The Queue module implements multi-producer, multi-consumer queues and 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 and it depends on the availability of thread support in Python.

This module implements three types of queue, which differ only in the order in which the entries are retrieved. For FIFO queue, the first tasks added are the first retrieved and for LIFO queue, the most recently added entry is the first retrieved (operating like a stack). And for priority queue, the entries are kept sorted (using the heapq module) and the lowest valued entry is retrieved first.

This queue module defines the following classes and exceptions.

class Queue.Queue(maxsize=0)

This is a constructor for a FIFO queue. Argument maxsize is an integer that sets the upper bound 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 then the queue size will be infinite.

class Queue.LifoQueue(maxsize=0)

This is constructor for a LIFO queue. Argument 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 then the queue size will be infinite.

class Queue.PriorityQueue(maxsize=0)

This is constructor for a priority queue. Argument 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 then the queue size is infinite.

exception Queue.Empty

This line indicate that the exception raised when non-blocking get() (or get_nowait()) is called on a Queue object which is empty.

exception Queue.Full

This line indicate that the exception raised when non-blocking put() (or put_nowait()) is called on a Queue object which is full.

Queue Objects

Queue.qsize()

This function returns the approximate size of the queue.

Queue.empty()

This function returns True if the queue is empty otherwise False. 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()

Returns 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[, timeout]])

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.get([block[, timeout]])

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).

Queue.task_done()

Indicates 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 code

import queue
#maximum capacity of queue is 20
Q = queue.Queue(maxsize=40)
Q.put(50)
Q.put(90)
Q.put(10)
Q.put(70)
print(Q.get())
print(Q.get())
print(Q.get())
print(Q.get())

Output

50
90
10
70

Example of Underflow/Overflow

import queue
Q = queue.Queue(maxsize=30)
print(Q.qsize())
Q.put(50)
Q.put(90)
Q.put(10)
Q.put(70)
print("Full: ", Q.full())
Q.put(90)
Q.put(100)
print("Full: ", Q.full())
print(Q.get())
print(Q.get())
print(Q.get())
print("Empty: ", Q.empty())
print(Q.get())
print(Q.get())
print(Q.get())
print("Empty: ", Q.empty())
print("Full: ", Q.full())

Output

0
Full: False
Full: False
50
90
10
Empty: False
70
90
100
Empty: True
Full: False

Example3 of Stack

import queue
S = queue.LifoQueue(maxsize=10)
# qsize() give the maxsize of
# the Queue
print(S.qsize())
S.put(50)
S.put(90)
S.put(10)
S.put(70)
S.put(90)
S.put(10)
print("Full: ", S.full())
print("Size: ", S.qsize())
# Data will be accessed in the
# reverse order Reverse of that
# of Queue
print(S.get())
print(S.get())
print(S.get())
print(S.get())
print(S.get())
print("Empty: ", S.empty())

Output

0
Full: False
Size: 6
10
90
70
10
90
Empty: False