What is the fastest way to merge two lists in python?

Question

Given,

list_1 = [1,2,3,4]
list_2 = [5,6,7,8]

What is the fastest way to achieve the following in python?

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Answer 1

If you are using python 3, there is one more way to do this and a little bit faster (tested only on python 3.7)

[*list1, *list2]

Benchmark

from timeit import timeit
x = list(range(10000))
y = list(x)

def one():
    x + y

def two():
    [*x, *y]

print(timeit(one, number=1000, globals={'x':x, 'y': y}))
print(timeit(two, number=1000, globals={'x':x, 'y': y}))
0.10456193100253586
0.09631731400440913

Answer 2

I tested out several ways to merge two lists (see below) and came up with the following order after running each several times to normalize the cache changes (which make about a 15% difference).

import time
c = list(range(1,10000000))
c_n = list(range(10000000, 20000000))
start = time.time()
*insert method here* 
print (time.time()-start)

• Method 1: c.extend(c_n)

  • Representative result: 0.11861872673034668

• Method 2: c += c_n

  • Representative result: 0.10558319091796875

• Method 3: c = c + c_n

  • Representative result: 0.25804924964904785

• Method 4: c = [*c, *c_n]

  • Representative result: 0.22019600868225098

Conclusion Use += or .extend() if you want to merge in place. They are significantly faster.

Answer 3

list_1 + list_2 does it. Example -

>>> list_1 = [1,2,3,4]
>>> list_2 = [5,6,7,8]
>>> list_1 + list_2
[1, 2, 3, 4, 5, 6, 7, 8]

Answer 4

a=[1,2,3]
b=[4,5,6]

c=a+b
print(c)

OUTPUT:

 >>> [1, 2, 3, 4, 5, 6]

In the above code "+" operator is used to concatenate the 2 lists into a single list.

ANOTHER SOLUTION:

 a=[1,2,3]
 b=[4,5,6]
 c=[] #Empty list in which we are going to append the values of list (a) and (b)

 for i in a:
     c.append(i)
 for j in b:
     c.append(j)

 print(c)

OUTPUT:

>>> [1, 2, 3, 4, 5, 6]

Answer 5

You can just use concatenation:

list = list_1 + list_2

If you don't need to keep list_1 around, you can just modify it:

list_1.extend(list_2)