Course 5.1Lesson 2 of 5

Changing a list

A list can be changed. append adds an item at the end, insert puts one at a position, remove deletes the first match, pop takes the last item out and gives it back, and a position on the left of an equals sign replaces an item. sort puts the items in order and reverse turns them around, and both change the list itself and give back None. sorted gives a new sorted list and keeps the original. A string cannot be changed, and trying to do so gives a TypeError.

about 35 min1 simulation2 practice questions

Lesson 2 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

add, insert, remove and replace items,
put a list in order, with sort() and with sorted(),
say which actions change the list itself, and
read the error when you try to change a string.

Lists can change

A string is fixed: you can make a new string, but you cannot change one letter of the old one. A list is not like that. We can add items, remove items, replace them and put them in order. These actions are called methods, and we write them after a dot, as we did for strings.

Four things that a list can do, each starting from the list 3, 1, 2. append of 4 gives 3, 1, 2, 4. remove of 1 gives 3, 2. sort gives 1, 2, 3. reverse gives 2, 1, 3.

Method	What it does
`a.append(x)`	adds `x` at the end
`a.insert(i, x)`	puts `x` at position `i`, and the others move along
`a.remove(x)`	deletes the first `x`
`a.pop()`	takes the last item out and gives it back
`a.pop(i)`	takes the item at position `i` out and gives it back
`a.sort()`	puts the items in order, smallest first
`a.sort(reverse=True)`	puts them in order, biggest first
`a.reverse()`	turns the list around
`a[0] = 99`	replaces the item at position 0

The methods change the list itself

This is the point that surprises beginners. a.append(4) does not give a new list. It changes a. The same is true of sort, reverse, insert, remove and pop.

Here is a related thing: these methods give back None, because their work is the change, not an answer. A very common mistake is a = a.sort(). It sorts the list, and then sets a to None. After that, a is gone!

The rule:

a.sort() changes a, and gives back None.
sorted(a) gives a new list, and a stays.

A string has no sort(), because a string cannot be changed. A list can.

What a string cannot do

A string cannot be changed. Look.

The word for "can be changed" is mutable, and for "cannot be changed" it is immutable. Lists are mutable, and strings are immutable. You will meet the word again.

Removing items: three ways

a.remove("apple") removes the item with that value, the first one that it finds. It gives a ValueError if the value is not there.
a.pop(2) removes the item at position 2, and gives it back.
del a[2] also removes the item at position 2, and gives nothing back.

Think about which one you need: do you know the value, or the position?

Common mistakes

a = a.sort() or a = a.append(4). These give None. Write a.sort() on its own.
Removing items inside a loop over the same list. The loop skips items. Make a new list instead (next lesson).
remove() of a value that is not in the list. A ValueError. Ask with in first.
insert() with the arguments the wrong way round. The position comes first, then the value.
Trying to change a string. Make a new string.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab Change the list. You add, remove and sort, and the checker confirms the final list.
Build. Open the problem Sort the marks and the problem A shopping list.
Stretch. Open the problem A list can change, a string cannot and explain the difference in your own words.

Remember these

append, insert, remove, pop, sort and reverse change the list itself.
They give back None. Never write a = a.sort().
sorted(a) is a new sorted list. The original stays.
Lists are mutable. Strings are not.