Detailed explanation of the deque double-ended queue structure in Python's collections module-Python Tutorial-php.cn

Deque is the abbreviation of double-ended queue, similar to list, but provides insertion and deletion operations at both ends.

appendleft inserts
popleft pops the value on the left side of the list
extendleft expands on the left

For example:

queue = deque()
# append values to wait for processing
queue.appendleft("first")
queue.appendleft("second")
queue.appendleft("third")
# pop values when ready
process(queue.pop()) # would process "first"
# add values while processing
queue.appendleft("fourth")
# what does the queue look like now&#63;
queue # deque(['fourth', 'third', 'second'])

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As a double-ended queue, deque also provides some other useful methods, such as rotate, etc. Let’s take a look at them below:

Padding
Deque can be filled from either end, which is called "left end" and "right end" in python.

import collections
d1 = collections.deque()
d1.extend('abcdefg')
print 'extend:', d1
d1.append('h')
print 'append:', d1
d2 = collections.deque()
d2.extendleft(xrange(6))
print 'extendleft', d2
d2.appendleft(6)
print 'appendleft', d2

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extendleft() iteratively processes its input, completing the same processing as appendleft() for each element.

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])

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Use
Deque elements can be utilized from both ends, depending on the algorithm applied.

import collections
print "From the right:"
d = collections.deque('abcdefg')
while True:
 try:
  print d.pop(),
 except IndexError:
  break
print
print "\nFrom the left:"
d = collections.deque(xrange(6))
while True:
 try:
  print d.popleft(),
 except IndexError:
  break
print

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Use pop() to delete an element from the right end of deque, and use popleft() to delete an element from the left end of deque.

From the right:
g f e d c b a

From the left:
0 1 2 3 4 5

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Since deques are thread-safe, the contents of the queue can be utilized from both ends simultaneously in different threads.

import collections
import threading
import time
candle = collections.deque(xrange(5))
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()

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The thread processes both ends alternately, deleting elements until the deque is empty.

 Left: 0 Right: 4

 Right: 3 Left: 1

 Right: 2 Left done

 Right done

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Rotate
Another function of deque is that it can rotate in any direction and skip some elements.

import collections
d = collections.deque(xrange(10))
print 'Normal:', d
d= collections.deque(xrange(10))
d.rotate(2)
print 'Right roration:', d
d = collections.deque(xrange(10))
d.rotate(-2)
print 'Left roration:', d

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Result:

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

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Another example:

# -*- coding: utf-8 -*-
"""
下面这个是一个有趣的例子，主要使用了deque的rotate方法来实现了一个无限循环
的加载动画
"""
import sys
import time
from collections import deque
fancy_loading = deque('>--------------------')
while True:
 print '\r%s' % ''.join(fancy_loading),
 fancy_loading.rotate(1)
 sys.stdout.flush()
 time.sleep(0.08)

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Output result:

# 一个无尽循环的跑马灯 
------------->-------

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Related labels：

collections deque python deque

source：php.cn

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