Deque

A double-ended queue, or deque, supports adding and removing elements from either end. 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]

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

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

$ python collections_deque.py

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'])

Populating

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

import collections

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

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

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

$ python collections_deque_populating.py

extend    : deque(['a', 'b', 'c', 'd', 'e', 'f', 'g'])
append    : deque(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
extendleft: deque(['g', 'f', 'e', 'd', 'c', 'b', 'a'])
appendleft: deque(['h', 'g', 'f', 'e', 'd', 'c', 'b', 'a'])

Consuming

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

import collections

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

print '\nFrom the left:'
d = collections.deque('abcdefg')
while True:
    try:
        print d.popleft()
    except IndexError:
        break

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

$ python collections_deque_consuming.py

From the right:
g
f
e
d
c
b
a

From the left:
a
b
c
d
e
f
g

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(xrange(11))

def burn(direction, nextSource):
    while True:
        try:
            next = nextSource()
        except IndexError:
            break
        else:
            print '%8s: %s' % (direction, next)
            time.sleep(0.1)
    print '%8s done' % direction
    return

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

left.start()
right.start()

left.join()
right.join()

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

$ python collections_deque_both_ends.py

    Left: 0
   Right: 10
   Right: 9
     Left: 1
   Right: 8
    Left: 2
   Right: 7
    Left: 3
   Right: 6
    Left: 4
   Right: 5
    Left done
   Right done

Rotating

Another useful capability of the deque is to rotate it in either direction, to skip over some items.

import collections

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

d = collections.deque(xrange(10))
d.rotate(2)
print 'Right rotation:', d

d = collections.deque(xrange(10))
d.rotate(-2)
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.

$ python collections_deque_rotate.py

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])

See also

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