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A quick list of python front lists

What is the fastest way to expand the front of an array in python? Suppose I have 2 arrays: a and b. I want to make the fastest way a = b + a (b should not change).

My little benches:

test 1:

a,b = [],[]
for i in range(0,100000):
    a.append(i)
    b.append(i)

def f(a,b):
    for i in range(0,100):
        a=a+b

import cProfile
cProfile.run('f(a,b)')

time: ~ 12 s

test 2:

a,b = [],[]
for i in range(0,100000):
    a.append(i)
    b.append(i)

def f(a,b):
    for i in range(0,100):
        a[0:0] = b

import cProfile
cProfile.run('f(a,b)')

time: ~ 1.5 s

test3:

a,b = [],[]
for i in range(0,100000):
    a.append(i)
    b.append(i)

lenb = len(b)
def f(a,b):
    for i in range(0,100):
        b.extend(a)
        # do something with b
        b = b[:lenb]

import cProfile
cProfile.run('f(a,b)')

time: ~ 0.4 s

But I think it should be faster, because combining lists should be done as changing a few basic pointers. And the following code is the fastest, but b changes, and not (SO NOT GOOD FOR OUR PURPOSE): the "WRONG" test:

a,b = [],[]
for i in range(0,100000):
    a.append(i)
    b.append(i)

def f(a,b):
    for i in range(0,100):
        b.extend(a)

import cProfile
cProfile.run('f(a,b)')

time: ~ 0.13s

Thus, theoretically there should be a way to expand the front in time of the "WRONG" test.

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1 answer

collections.deque, , .appendleft .extendleft, - appendleft , ( deque), extendleft :

def extendleft(self, other)
    for item in other:
        self.appendleft(c)

a = b+a :

a.extendleft(reversed(b))
+10

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