Python - How To Append a List to Another List in Python

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This tutorial will demonstrate ways to append a list into an already existing list in Python.

Use the extend() Method to Append a List Into Another List in Python

Python has a built-in method for lists named extend() that accepts an iterable as a parameter and adds it into the last position of the current iterable. Using it for lists will append the list parameter after the last element of the main list.

For example, declare two lists and add the second list into the main list using the method extend().

lst = [4, 6, 8] lst2 = [10, 12, 14]  lst.extend(lst2) print(lst) 

Output:

[4, 6, 8, 10, 12, 14] 

The extend() method provides a straightforward way to append a list into an existing list with a simple function call.

Use chain() Function in the itertools Module to Append Into a List in Python

itertools is a Python module containing fast and efficient utility methods for iterables. This module has the function chain() that accepts a variable number of same-type iterables and concatenates them together in sequence based on the parameters.

We can use the chain() function to append multiple lists and form them into a single list.

For this example, declare three different lists and set them as parameters for the itertools.chain() function. We then wrap the function with another function list(), which initializes a single list from the return value of the chain() function.

import itertools  lst = [9, 8, 7] lst2 = [6, 5, 4] lst3 = [3, 2, 1]  lst_all = list(itertools.chain(lst, lst2, lst3))  print(lst_all) 

Output:

[9, 8, 7, 6, 5, 4, 3, 2, 1] 

Using itertools.chain(), the parameters can be as many or as few as you want, and you will be provided with an efficient way of concatenating lists together and forming them into a single list.

Use the Concatenation + Operator to Append Multiple Lists in Python

Another simple method of appending multiple lists together is to use the + operator, which supports list concatenation in Python.

Simply perform the concatenation + operation on existing list variables, and the output will be a single combined list in order of the operands inputted in the code.

lst = [1, 3, 5] lst2 = [2, 4, 6] lst3 = [0, 0, 0]  lst_all = lst + lst2 + lst3 print(lst_all) 

Output:

[1, 3, 5, 2, 4, 6, 0, 0, 0] 

In summary, three simple and efficient ways to append a list or multiple lists into a main list are extending, chaining, and using the concatenation + operator.

All three solutions perform reliably, and the comparative performance regarding time is relatively trivial, so it’s a matter of personal preference and convenience.

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