Use this :
#Inputs
L1 = [1, 2]
L2 = [3,4,5]
#Code
L1+L2
#Output
[1, 2, 3, 4, 5]
By using the (+) operator you can skip the multiple append & extend operators in just one line of code and this is valid for more then two of lists by L1+L2+L3+L4.......etc.
Happy Learning...:)