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Detailed explanation of the deque double-ended queue structure in Python's collections module
Detailed explanation of the deque double-ended queue structure in Python's collections module
Deque is the abbreviation of double-ended queue, similar to list, but provides insertion and deletion operations at both ends.
- appendleft inserts on the left side of the list
- 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?
queue # deque(['fourth', 'third', 'second'])
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
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])
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
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
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()
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
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
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])
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)
Output result:
# 一个无尽循环的跑马灯 ------------->-------
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