6 Ways to Concatenate Lists in Python

In this tutorial, we will unveil different methods to concatenate or combine together multiple lists in Python. Python Lists serve the purpose of storing homogeneous elements and perform manipulations on the same.

In general, Concatenation is the process of joining the elements of a particular data-structure in an end-to-end manner.

The following are the 6 ways to concatenate lists in Python.

1. concatenation (+) operator
2. Naive Method
3. List Comprehension
4. extend() method
5. ‘*’ operator
6. itertools.chain() method

Concatenation operator (+) for List Concatenation

The '+' operator can be used to concatenate two lists. It appends one list at the end of the other list and results in a new list as output.

Example:

list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 


res = list1 + list2 


print ("Concatenated list:\n" + str(res)) 

Output:

Concatenated list:
[10, 11, 12, 13, 14, 20, 30, 42]

Naive Method for List Concatenation

In the Naive method, a for loop is used to traverse the second list. After this, the elements from the second list get appended to the first list. The first list results out to be the concatenation of the first and the second list.

Example:

list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 

print("List1 before Concatenation:\n" + str(list1))
for x in list2 : 
    list1.append(x) 


print ("Concatenated list i.e. list1 after concatenation:\n" + str(list1)) 

Output:

List1 before Concatenation:
[10, 11, 12, 13, 14]
Concatenated list i.e. list1 after concatenation:
[10, 11, 12, 13, 14, 20, 30, 42]

List Comprehension to concatenate lists

Python List Comprehension is an alternative method to concatenate two lists in Python. List Comprehension is basically the process of building/generating a list of elements based on an existing list.

It uses for loop to process and traverses the list in an element-wise fashion. The below inline for-loop is equivalent to a nested for loop.

Example:

list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 

res = [j for i in [list1, list2] for j in i] 

print ("Concatenated list:\n"+ str(res)) 

Output:

Concatenated list:
 [10, 11, 12, 13, 14, 20, 30, 42]

Python extend() method for List Concatenation

Python’s extend() method can be used to concatenate two lists in Python. The extend() function does iterate over the passed parameter and adds the item to the list thus, extending the list in a linear fashion.

Syntax:

list.extend(iterable)

Example:

list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 
print("list1 before concatenation:\n" + str(list1))
list1.extend(list2) 
print ("Concatenated list i.e ,ist1 after concatenation:\n"+ str(list1)) 

All the elements of the list2 get appended to list1 and thus the list1 gets updated and results as output.

Output:

list1 before concatenation:
[10, 11, 12, 13, 14]
Concatenated list i.e ,ist1 after concatenation:
[10, 11, 12, 13, 14, 20, 30, 42]

Python ‘*’ operator for List Concatenation

Python’s '*' operator can be used to easily concatenate two lists in Python.

The ‘*’ operator in Python basically unpacks the collection of items at the index arguments.

For example: Consider a list my_list = [1, 2, 3, 4].

The statement *my_list would replace the list with its elements at the index positions. Thus, it unpacks the items of the lists.

Example:

list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 

res = [*list1, *list2] 
  
print ("Concatenated list:\n " + str(res)) 

In the above snippet of code, the statement res = [*list1, *list2] replaces the list1 and list2 with the items in the given order i.e. elements of list1 after elements of list2. This performs concatenation and results in the below output.

Output:

Concatenated list:
 [10, 11, 12, 13, 14, 20, 30, 42]

Python itertools.chain() method to concatenate lists

Python itertools modules’ itertools.chain() function can also be used to concatenate lists in Python.

The itertools.chain() function accepts different iterables such as lists, string, tuples, etc as parameters and gives a sequence of them as output.

It results out to be a linear sequence. The data type of the elements doesn’t affect the functioning of the chain() method.

For example: The statement itertools.chain([1, 2], [‘John’, ‘Bunny’]) would produce the following output: 1 2 John Bunny

Example:

import itertools
list1 = [10, 11, 12, 13, 14] 
list2 = [20, 30, 42] 

res = list(itertools.chain(list1, list2)) 
   
  
print ("Concatenated list:\n " + str(res)) 

Output:

Concatenated list:
 [10, 11, 12, 13, 14, 20, 30, 42]

Conclusion

Thus, in this article, we have understood and implemented different ways of Concatenating lists in Python.