Python Lists

Lists are an essential Python data structure.

The allow you to group together multiple values and reference them all with a common name.

For example:

dogs = ["Roger", "Syd"]

A list can hold values of different types:

items = ["Roger", 1, "Syd", True]

You can check if an item is contained into a list with the in operator:

print("Roger" in items) # True

A list can also be defined as empty:

items = []

You can reference the items in a list by their index, starting from zero:

items[0] # "Roger"
items[1] # 1
items[3] # True

Using the same notation you can change the value stored at a specific index:

items[0] = "Roger"

You can also use the index() method:

items.index("Roger") # 0
items.index("Syd") # 2

As with strings, using a negative index will start searching from the end:

items[-1] # True

You can also extract a part of a list, using slices:

items[0:2] # ["Roger", 1]
items[2:] # ["Syd", True]

Get the number of items contained in a list using the len() global function, the same we used to get the length of a string:

len(items) #4

You can add items to the list by using a list append() method:

items.append("Test")

or the extend() method:

items.extend(["Test"])

You can also use the += operator:

items += ["Test"]

# items is ['Roger', 1, 'Syd', True, 'Test']

Tip: with extend() or += don’t forget the square brackets. Don’t do items += "Test" or items.extend("Test") or Python will add 4 individual characters to the list, resulting in ['Roger', 1, 'Syd', True, 'T', 'e', 's', 't']

Remove an item using the remove() method:

items.remove("Test")

You can add multiple elements using

items += ["Test1", "Test2"]

#or

items.extend(["Test1", "Test2"])

These append the item to the end of the list.

To add an item in the middle of a list, at a specific index, use the insert() method:

items.insert(1, "Test") # add "Test" at index 1

To add multiple items at a specific index, you need to use slices:

items[1:1] = ["Test1", "Test2"]

Sort a list using the sort() method:

items.sort()

Tip: sort() will only work if the list holds values that can be compared. Strings and integers for example can’t be compared, and you’ll get an error like TypeError: '<' not supported between instances of 'int' and 'str' if you try.

The sort() methods orders uppercase letters first, then lowercased letters. To fix this, use:

items.sort(key=str.lower)

instead.

Sorting modifies the original list content. To avoid that, you can copy the list content using

itemscopy = items[:]

or use the sorted() global function:

print(sorted(items, key=str.lower))

that will return a new list, sorted, instead of modifying the original list.