Queue in Python
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- Difficulty Level : Easy
- Last Updated : 10 Jan, 2023
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Like stack, queue is a linear data structure that stores items in First In First Out (FIFO) manner. With a queue the least recently added item is removed first. A good example of queue is any queue of consumers for a resource where the consumer that came first is served first.
Operations associated with queue are:
- Enqueue: Adds an item to the queue. If the queue is full, then it is said to be an Overflow condition – Time Complexity : O(1)
- Dequeue: Removes an item from the queue. The items are popped in the same order in which they are pushed. If the queue is empty, then it is said to be an Underflow condition – Time Complexity : O(1)
- Front: Get the front item from queue – Time Complexity : O(1)
- Rear: Get the last item from queue – Time Complexity : O(1)
Implementation
There are various ways to implement a queue in Python. This article covers the implementation of queue using data structures and modules from Python library.
Queue in Python can be implemented by the following ways:
- list
- collections.deque
- queue.Queue
Implementation using list
List is a Python’s built-in data structure that can be used as a queue. Instead of enqueue() and dequeue(), append() and pop() function is used. However, lists are quite slow for this purpose because inserting or deleting an element at the beginning requires shifting all of the other elements by one, requiring O(n) time.
Python3
# Python program to
# demonstrate queue implementation
# using list
# Initializing a queue
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 0Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 1 Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 2
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 4
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 9Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 2Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 7Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 0
Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 2
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 8Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 90 Full: True Elements dequeued from the queue a b c Empty: True Empty: False Full: False 0
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 8Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 90 Full: True Elements dequeued from the queue a b c Empty: True Empty: False Full: False 0
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 8Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 90 Full: True Elements dequeued from the queue a b c Empty: True Empty: False Full: False 0
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6# Python program to 2Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque 0
# Python program to 7
# Python program to 8
# Python program to 9
Output:
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list
Implementation using collections.deque
Queue in Python can be implemented using deque class from the collections module. Deque is preferred over list in the cases where we need quicker append and pop operations from both the ends of container, as deque provides an O(1) time complexity for append and pop operations as compared to list which provides O(n) time complexity. Instead of enqueue and deque, append() and popleft() functions are used.
Python3
# demonstrate queue implementation0
# demonstrate queue implementation
# demonstrate queue implementation2
# demonstrate queue implementation5 # demonstrate queue implementation6# demonstrate queue implementation7 # demonstrate queue implementation8
# Initializing a queue
# using list1Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 1 # using list3
# using list5
# using list6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
# using list6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 9Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
# using list6Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 2Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 7Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5# Initializing a queue1
# Initializing a queue3
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6# Initializing a queue6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5# Initializing a queue9
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5# Initializing a queue9
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5# Initializing a queue9
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6# Python program to 2Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5# Initializing a queue1
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 12
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 13
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 14
Output:
Traceback (most recent call last): File "/home/b2fa8ce438c2a9f82d6c3e5da587490f.py", line 23, in q.popleft() IndexError: pop from an empty deque
Implementation using queue.Queue
Queue is built-in module of Python which is used to implement a queue. queue.Queue(maxsize) initializes a variable to a maximum size of maxsize. A maxsize of zero ‘0’ means a infinite queue. This Queue follows FIFO rule.
There are various functions available in this module:
- maxsize – Number of items allowed in the queue.
- empty() – Return True if the queue is empty, False otherwise.
- full() – Return True if there are maxsize items in the queue. If the queue was initialized with maxsize=0 (the default), then full() never returns True.
- get() – Remove and return an item from the queue. If queue is empty, wait until an item is available.
- get_nowait() – Return an item if one is immediately available, else raise QueueEmpty.
- put(item) – Put an item into the queue. If the queue is full, wait until a free slot is available before adding the item.
- put_nowait(item) – Put an item into the queue without blocking. If no free slot is immediately available, raise QueueFull.
- qsize() – Return the number of items in the queue.
Python3
# demonstrate queue implementation0
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 16
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 17
# demonstrate queue implementation5 Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 0# demonstrate queue implementation7 Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 23
# Initializing a queue
# using list1Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 1 Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 28Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 1 Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 30Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 33
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 34
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 36
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 38
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 39Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 39Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 9Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 39Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 2Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 49
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 50
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 53Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 54
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 56
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6# Initializing a queue6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 62
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 62
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 62
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 68
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 50
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 72Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 73
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 39Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 76Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 7
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 72Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 81
Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 5Initial queue deque(['a', 'b', 'c']) Elements dequeued from the queue a b c Queue after removing elements deque([]) 6Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 84Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 85
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 87
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 88
Traceback (most recent call last): File "/home/ef51acf025182ccd69d906e58f17b6de.py", line 25, in print(queue.pop(0)) IndexError: pop from empty list 89
Output:
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