Python | Insert list in another list

The problem of inserting a number at any index is a quite common one. But sometimes we require to insert the whole list into another list. These kinds of problems occur in Machine Learning while playing with data. Let’s discuss certain ways in which this problem can be solved.

Method #1 : Using insert() + loop
In this method, we insert one element by 1 at a time using the insert function. This way we add all the list elements at the specified index in other list.

# Python3 code to demonstrate  
# to insert one list in another 
# using insert() + loop 
  
# initializing lists  
test_list = [4, 5, 6, 3, 9] 
insert_list = [2, 3] 
  
# initializing position 
pos = 2
  
# printing original list 
print ("The original list is : " + str(test_list)) 
  
# printing insert list  
print ("The list to be inserted is : " + str(insert_list)) 
  
# using insert() + loop 
# to insert one list in another 
for i in range(len(insert_list)): 
    test_list.insert(i + pos, insert_list[i]) 
  
# printing result  
print ("The list after insertion is : " +  str(test_list)) 

Output :

The original list is : [4, 5, 6, 3, 9]
The list to be inserted is : [2, 3]
The list after insertion is : [4, 5, 2, 3, 6, 3, 9]

Method #2 : Using list slicing
This is the most pythonic and elegant way to perform this particular task. In this method, we just slice the list where we need to add the element and assign the list to be inserted.

# Python3 code to demonstrate  
# to insert one list in another 
# using list slicing 
  
# initializing lists  
test_list = [4, 5, 6, 3, 9] 
insert_list = [2, 3] 
  
# initializing position 
pos = 2
  
# printing original list 
print ("The original list is : " + str(test_list)) 
  
# printing insert list  
print ("The list to be inserted is : " + str(insert_list)) 
  
# using list slicing 
# to insert one list in another 
test_list[pos:pos] = insert_list 
  
# printing result  
print ("The list after insertion is : " +  str(test_list)) 

Output :

The original list is : [4, 5, 6, 3, 9]
The list to be inserted is : [2, 3]
The list after insertion is : [4, 5, 2, 3, 6, 3, 9]

My Personal Notes

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