deque — Double-Ended Queue

A double-ended queue, or deque, supports adding and removing elements from either end of the queue. The more commonly used stacks and queues are degenerate forms of deques, where the inputs and outputs are restricted to a single end.
import collections

d = collections.deque('abcdefg')
print('Deque:', d)
print('Length:', len(d))
print('Left end:', d[0])
print('Right end:', d[-1])

print('remove(c):', d)

Since deques are a type of sequence container, they support some of the same operations as list, such as examining the contents with __getitem__(), determining length, and removing elements from the middle of the queue by matching identity.

$ python3

Deque: deque(['a', 'b', 'c', 'd', 'e', 'f', 'g'])
Length: 7
Left end: a
Right end: g
remove(c): deque(['a', 'b', 'd', 'e', 'f', 'g'])


A deque can be populated from either end, termed “left” and “right” in the Python implementation.
import collections

# Add to the right
d1 = collections.deque()
print('extend    :', d1)
print('append    :', d1)

# Add to the left
d2 = collections.deque()
print('extendleft:', d2)
print('appendleft:', d2)

The extendleft() function iterates over its input and performs the equivalent of an appendleft() for each item. The end result is that the deque contains the input sequence in reverse order.

$ python3

extend    : deque(['a', 'b', 'c', 'd', 'e', 'f', 'g'])
append    : deque(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
extendleft: deque([5, 4, 3, 2, 1, 0])
appendleft: deque([6, 5, 4, 3, 2, 1, 0])


Similarly, the elements of the deque can be consumed from both ends or either end, depending on the algorithm being applied.
import collections

print('From the right:')
d = collections.deque('abcdefg')
while True:
        print(d.pop(), end='')
    except IndexError:

print('\nFrom the left:')
d = collections.deque(range(6))
while True:
        print(d.popleft(), end='')
    except IndexError:

Use pop() to remove an item from the “right” end of the deque and popleft() to take an item from the “left” end.

$ python3

From the right:
From the left:

Since deques are thread-safe, the contents can even be consumed from both ends at the same time from separate threads.
import collections
import threading
import time

candle = collections.deque(range(5))

def burn(direction, nextSource):
    while True:
            next = nextSource()
        except IndexError:
            print('{:>8}: {}'.format(direction, next))
    print('{:>8} done'.format(direction))

left = threading.Thread(target=burn,
                        args=('Left', candle.popleft))
right = threading.Thread(target=burn,
                         args=('Right', candle.pop))



The threads in this example alternate between each end, removing items until the deque is empty.

 $ python3

 Left: 0
Right: 4
Right: 3
 Left: 1
Right: 2
 Left done
Right done


Another useful aspect of the deque is the ability to rotate it in either direction, so as to skip over some items.
import collections

d = collections.deque(range(10))
print('Normal        :', d)

d = collections.deque(range(10))
print('Right rotation:', d)

d = collections.deque(range(10))
print('Left rotation :', d)

Rotating the deque to the right (using a positive rotation) takes items from the right end and moves them to the left end. Rotating to the left (with a negative value) takes items from the left end and moves them to the right end. It may help to visualize the items in the deque as being engraved along the edge of a dial.

$ python3

Normal        : deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
Right rotation: deque([8, 9, 0, 1, 2, 3, 4, 5, 6, 7])
Left rotation : deque([2, 3, 4, 5, 6, 7, 8, 9, 0, 1])

Constraining the Queue Size

A deque instance can be configured with a maximum length so that it never grows beyond that size. When the queue reaches the specified length, existing items are discarded as new items are added. This behavior is useful for finding the last n items in a stream of undetermined length.
import collections
import random

# Set the random seed so we see the same output each time
# the script is run.

d1 = collections.deque(maxlen=3)
d2 = collections.deque(maxlen=3)

for i in range(5):
    n = random.randint(0, 100)
    print('n =', n)
    print('D1:', d1)
    print('D2:', d2)

The deque length is maintained regardless of which end the items are added to.

$ python3

n = 17
D1: deque([17], maxlen=3)
D2: deque([17], maxlen=3)
n = 72
D1: deque([17, 72], maxlen=3)
D2: deque([72, 17], maxlen=3)
n = 97
D1: deque([17, 72, 97], maxlen=3)
D2: deque([97, 72, 17], maxlen=3)
n = 8
D1: deque([72, 97, 8], maxlen=3)
D2: deque([8, 97, 72], maxlen=3)
n = 32
D1: deque([97, 8, 32], maxlen=3)
D2: deque([32, 8, 97], maxlen=3)

See also

  • Wikipedia: Deque – A discussion of the deque data structure.
  • Deque Recipes – Examples of using deques in algorithms from the standard library documentation.