Python Lists

This content originally appeared on flaviocopes.com and was authored by flaviocopes.com

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(0) # "Roger"
items.index(1) # 1

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("Test", 1) # 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.

This content originally appeared on flaviocopes.com and was authored by flaviocopes.com

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