Extending Python Lists

In this week's Python snippet post we're looking at the extend method for lists.

extend is a lot like the append method, but instead of adding a single value, extend allows us to append several elements to the end of a given list object.

Let's start by defining a couple of lists:

l_1 = [1, 2, 3, 4]
l_2 = [5, 6, 7, 8]

We're going to use extend to add the values from l_2 onto the end of l_1:

l_1.extend(l_2)

print(l_1)  # [1, 2, 3, 4, 5, 6, 7, 8]

As we can see, the extend method in an in-place operation, so it modifies the original list. Other than that, it perform very similarly to using the + operator with lists, so why should we care about extend?

Well, extend can accept any iterable, while using something like + to perform concatenation only works when both objects are lists. So we can do this:

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

While the following will give us a TypeError:

[1, 2, 3] + (4, 5, 6)

Using extend on the other hand, everything works just fine:

l_1 = [1, 2, 3, 4]
t_1 = (5, 6, 7, 8)

l_1.extend(t_1)  # [1, 2, 3, 4, 5, 6, 7, 8]

Wrapping up

That's it for extend! I hope you learnt something new, and I hope you can find places to use extend in your own code.

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