How to Concatenate Lists in Python

Python allows us to easily concatenate two or more lists using the + operator. See the following example:

list_one = [0, 1, 2]
list_two = [3, 4, 5]

new_list = list_one + list_two

Output

>>> new_list
[0, 1, 2, 3, 4, 5]

It should be noted that neither list_one nor list_two are affected by this operation. Rather the contents of each list are joined together to form a new list.

When you're dealing with potentially large lists and you need to be conscious of memory usage, you might consider using a Python generator instead. The + operator will concatenates lists, but it will create a new list in memory. If the goal is to process elements from the joined list, we can achieve this with a generator.

Suppose we have two giant lists: giant_list_one and giant_list_two. I won't initialize the lists in the example, but assume they take up a lot of memory.

import itertools

for item in itertools.chain(giant_list_one, giant_list_two):
    # operate on each item

The itertools library is handy for memory efficient looping. itertools.chain is a generator function that returns each element of the first list until it is exhausted, then returns each element from the next list, and the method can actually take any number of iterables.

Sometimes we want to merge lists, but return only unique values. If this is the case, we probably don't care about order, and we can use sets to deduplicate items.

fruit = ['apple', 'banana', 'orange']
more_fruit = ['peach', 'apple', 'banana']

unique_fruit = list(set(fruit + more_fruit))

Output

>>> unique_fruit
['banana', 'orange', 'apple', 'peach']

Notice, how we've discarded duplicate items from the list. If you can deconstruct the example, what we've done is simply:

• Create a new list with all elements from both lists (using the + operator).
• Create a set from the new list, which will automatically remove duplicate items (by definition).
• Create a final list from our set.

With the acceptance of PEP 448, implemented in Python 3.5+, you can also use the iterable unpacking operator: *. See the following example:

list_one = [0, 1, 2]
list_two = [3, 4, 5]

new_list = [*list_one, *list_two]

Output

>>> new_list
[0, 1, 2, 3, 4, 5]

One of the benefits of using this method is that you can merge any iterables, not just lists.

new_list = [*some_list, *some_tuple, *some_generator]

In my opinion, this is much more readable than using the + operator:

new_list = some_list + list(some_tuple) + list(some_generator)