Python | Add list at beginning of list

Last Updated : 05 Sep, 2019

Sometimes, while working with Python list, we have a problem in which we need to add a complete list to other. The rear end addition of list has been discussed before. But sometimes, we need to perform an append at beginning of list. Let’s discuss certain ways in which this task can be performed.

Method #1 : Using "+" operator
The “+” operator can be used to perform this particular task. In this, we just perform the addition of one list before other and construct a new list or perform the addition to same list.

# Python3 code to demonstrate working of
# Adding list at beginning of list
# using "+" operator
  
# initialize list
test_list = [1, 4, 5, 7, 6]
  
# initialize add list
add_list = [3, 4, 2, 10]
  
# printing original list
print("The original list is : " + str(test_list))
  
# printing add list 
print("The add list is : " + str(add_list))
  
# Adding list at beginning of list
# using "+" operator
test_list = add_list + test_list
  
# printing result
print("The original updated list is : " + str(test_list))

Output :

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

Method #2 : Using deque.extendleft() + reversed()
This task can also be performed using combination of above methods. In this, we just convert the list into a dequeue, to allow a front append, and then one by one addition is done by extendleft(), the add list is reversed so that addition take place in correct order using reversed().

# Python3 code to demonstrate working of
# Adding list at beginning of list
# using deque.extendleft() + reversed()
from collections import deque
  
# initialize list
test_list = [1, 4, 5, 7, 6]
  
# initialize add list
add_list = [3, 4, 2, 10]
  
# printing original list
print("The original list is : " + str(test_list))
  
# printing add list 
print("The add list is : " + str(add_list))
  
# Adding list at beginning of list
# using deque.extendleft() + reversed()
res = deque(test_list)
res.extendleft(reversed(add_list))
  
# printing result
print("The original updated list is : " + str(list(res)))

Output :

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

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Python | Add list at beginning of list