Lists, Part 2

Adding, Removing, Changing, and Finding Items in lists cheat sheet

action method returns possible errors
check length len(my_list) int
add: to the end my_list.append(item) -
insert: at position my_list.insert(pos, item) -
update: at position my_list[pos] = item - - IndexError if pos is >= len(my_list)
extend: add items from another list my_list.extend(other_list) -
is item in list? item in my_list True or False
index of item my_list.index(item) int ValueError if item is not in my_list
count of item my_list.count(item) int
remove an item my_list.remove(item) - ValueError if item not in my_list
remove the last item, or an item at an index my_list.pop() or my_list.pop(pos) item IndexError if pos >= len(my_list)

Checking Length

Before we add or remove items, it’s usually a good idea to check a list’s length. We do that with the len built in function. We can even use the len built in function to check the lengths of other types, like strings.

Let’s see it in action on a names list with two items, and a name string with four characters.

>>> len(names)
2
>>> name = "Nina"
>>> len(name)
4

Adding Items

Let’s start with a list of two names.

>>> names = ["Nina", "Max"]
my_list.append(item) adds to the end of my_list

We can use my_list.append(item) to add an additional item to the end of the list.

>>> names.append("John")
>>> names
['Nina', 'Max', 'John']
my_list.insert(pos, item) inserts an item into my_list at the given position

Use my_list.insert(pos, item() to insert items in an arbitrary position in the list. If the position is 0, we’ll insert at the beginning of the list.

>>> names.insert(0, "Rose")
>>> names
['Rose', 'Nina', 'Max', 'John']

You can call dir() on our names list to verify that it’s actually of type list. If you forget which order insert is called in, don’t forget you can always use the help() function on the REPL. Remember: Press q to quit the help screen. Let’s try it now:

>>> type(names)
<class 'list'>
>>> help(names.insert)

Help on method_descriptor:

insert(self, index, object, /)
    Insert object before index.

You can also call help on list.insert. Because names is already of type list, it achieves the same result.

my_list.extend(other_list) adds all the contents of other_list to my_list
>>> names = ["Nina", "Max"]
>>> colors = ["Red", "Blue"]
>>> names
['Nina', 'Max']
>>> names.extend(colors)
>>> names
['Nina', 'Max', 'Red', 'Blue']

Looking for Items

Looking for items in a list is slow. Each item needs to be checked in order to find a match.

This doesn’t matter much when you’re just getting started, unless your data set is large, or if you’re building high-performance systems. If you want to quickly search for an item, you’ll need to use a set or a dictionary instead.

There are a few ways to determine if an item is in the list, and at which position. Let’s try this on our list of names.

names = ["Nina", "Max", "Phillip", "Nina"]
Use the in keyword to determine if an item is present or not.
>>> "Nina" in names
True
>>> "Rose" in names
False
Use the my_list.index(item) method to find the first index of a potential match.

Notice that only the first index of the string "Nina" is returned. We’ll learn more about what an index is in the next chapter.

If the item we’re looking for is not in the list, Python with throw a ValueError.

You’ll learn how to deal with exceptions later. For now, you can use the in operator to check if an item is present in the list before finding its index.

>>> names.index("Nina")
0
>>> names.index("Rose")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 'Rose' is not in list
Use the my_list.count(item) method to find out how many times an item appears in a list.
>>> names.count("Nina")
2
>>> names.count("Rose")
0

Updating Items

To update items in a list, use the position of the item you’d like to change using square bracket [] syntax. Like: my_list[pos] = new_item

For example:

>>> names = ["Nina", "Max"]
>>> names[0] = "Rose"
>>> names
['Rose', 'Max']

Or, when used with my_list.index(item):

>>> names = ["Nina", "Max"]
>>> pos = names.index("Max")
>>> names[pos] = "Rose"
>>> names
['Nina', 'Rose']

You’ll see a IndexError: list assignment index out of range if you try to update an item in a position that doesn’t exist, that is if the position is greater than or equal to >= the length of the list.

>>> names = ["Nina", "Max"]
>>> len(names)
2
>>> names[2] = "Rose"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range

Removing Items

There are a few ways to remove items from a list.

Use my_list.remove(item) to remove the first instance of the item

Be careful. remove() only removes the first instance of the item from the list, which isn’t always what we want to do.

>>> names = ["Nina", "Max", "Rose"]
>>> names.remove("Nina")
>>> names
['Max', 'Rose']
>>>
>>>
>>> names = ["Nina", "Max", "Nina"]
>>> names.remove("Nina")
>>> names
['Max', 'Nina']

If we try to remove an item that’s not in the list, we’ll get a ValueError: list.remove(x): x not in list.

>>> names = ["Nina"]
>>> names.remove("Max")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list
Use my_list.pop() to remove the last item, or my_list.pop(index) to remove the item at that index

Using pop() will also return the item that was in that position. That’s useful if we want to save the item.

>>> names = ["Nina", "Max", "Rose"]
>>> names.pop()
'Rose'
>>> names
['Nina', 'Max']
>>> names.pop(1)
'Max'
>>> names
['Nina']

If we try to pop an item from an index that is longer than or equal to the length of the list, we’ll get an IndexError: pop index out of range.

>>> names = ["Nina"]
>>> len(names)
1
>>> names.pop(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: pop index out of range