Python's lists: append vs extend methods

Append vs Extend

Let's discover how to change lists.

What are lists in Python?

Lists are one of the fondamental tools of Python. They are what arrays are for other languages.
You can create a list simply like this:

mylist = [1,2,3]

But you can put everything in a list: strings, objects... anything.

names_list = ["John", "Peter", "Mary"]

There are different ways to add elements to a list. Let's take a look at  them.

Two different methods...

[caption id="" align="alignnone" width="715"] extend vs append[/caption]

With append you can append a single element that will extend the list:

>>> a = [1,2]
>>> a.append(3)
>>> a
[1,2,3]

If you want to extend more than one element you should use extend, because you can only append one elment or one list of element:

>>> a.append([4,5])
>>> a
>>> [1,2,3,[4,5]]

So that you get a nested list
Instead with extend you can extend a single element like this

>>> a = [1,2]
>>> a.extend([3])
>>> a
[1,2,3]

Or, differently from append, extend more elements in one time without nesting the list into the original one (that's the reason of the name extend)

>>> a.extend([4,5,6])
>>> a
[1,2,3,4,5,6]

Adding one element with both methods

append 1 element

>>> x = [1,2]
>>> x.append(3)
>>> x
[1,2,3]

extend one element

>>> x = [1,2]
>>> x.extend([3])
>>> x
[1,2,3,4]

Adding more elements... with different results

If you use append for more than one element, you have to pass a list of elements as arguments and you will obtain a NESTED list!

>>> x = [1,2]
>>> x.append([3,4])
>>> x
[1,2,[3,4]]

With extend, instead, you pass a list as argument, but you will obtain a list with the new element that are not nested in the old one.

>>> z = [1,2] 
>>> z.extend([3,4])
>>> z
[1,2,3,4]

So, with more elements, you will use extend to get a list with more items. You will use append, to append not more elements to the list, but one element that is a nested list as you can clearly see in the output of the code.