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← Functions   |   Loops →

7. Lists

Remember lists? They look like this:

books = ['Gender Trouble', 'Cruising Utopia', 'Living a Feminist Life']

For now, let's just create a list and print it out. In a text editor, our script will look like this:

books = ['Gender Trouble', 'Cruising Utopia', 'Living a Feminist Life']
print(books)

Save this to a new file called loop.py and run it with python loop.py. You should see the list printed out in the terminal.

So far, we've only learned one function: type(). Let's try out another, called len(), which returns the number of items in a list or the number of characters in a string.

First, let's "comment out" the print(books) statement with the hastag #, which tells python to ignore that line of code. Then, type out the following lines in your loop.py file:

books = ['Gender Trouble', 'Cruising Utopia', 'Living a Feminist Life']
# print(books)

list_length = len(book)

print(list_length)

Let's take apart this unfamiliar line of code: list_length = len(book)

  • First, we have saved a list of books to the variable, books.
  • Second, look to the next line this statement. list_length = len(book) takes the books variable from the previous line as an argument (or input data) for the len() function. That's why books is within the parenthesis. This syntax means that python will run the len() function on the items in books. Then, it sets the result of that process to a new variable, called list_length.
  • Finally, we print the list_length value.

This might appear a bit complex at first, but if you read the line slowly you should be able to connect the dots.

Notice that when you run the code above, you don't see the books list printed out. That's because that line has become a comment. If you put a # (hash or pound) at the beginning of a line, that line will be ignored.

List Indexing

A useful property of a list is the list index. This allows you to pick out an item from within the list by a number starting from zero:

print(books[0]) # Gender Trouble
print(books[1]) # Cruising Utopia

Note that the first item in the list is item[0]. The second item is item[1]. That's because counting in Python, and in almost all programming languages, starts from 0.

Additionally, you can print out the last item in a list using negative numbers, where -1 denotes the last item in the list:

print(books[-1]) # Living a Feminist Life

Slicing Lists

There are many things you can do with list indexing, like slicing. Slicing consists of taking a section of a list, using the list index to pick out a range of list items. For example, you could take out the first two items of a list with a slice that begins with 0 and ends with 2.

The slice syntax consists of square brackets, start point and end point, and a colon to indicate the gap in between. This should print out the first two items of your list.

print(books[0:2])

Note a couple of things. First, the start point is inclusive, meaning that Python will include the [0] item in your range, and the end point is exclusive, so Python won't print the [2] item. Instead, it will print everything up until that [2] item.

For ultimate brevity, you can also write this expression as:

print(books[:2])

The empty value before the colon allows Python to assume the range starts at the first list item, at [0]. You can also end the slice with :, if you want the list range to include all subsequent items until the end of the list. The example below will print everything from the second item to the end of the list.

print(books[1:])

With a list that contains three items total, list slicing might not seem very impressive right now. However, this will become a powerful tool once we get to more sophisticated text analysis and start to encounter lists that contain hundreds (or thousands!) of items.

Challenge

Create a new list of books in the REPL, with at least 5 books in your list. Make sure the total number of books in the list is an odd number. How do you get python to print out the book in the middle of the list? What about the three books in the middle? Remember that the first value in a slice is inclusive, and the final value is exclusive.

Solution

>>> books = ['Gender Trouble', 'Cruising Utopia', 'Living a Feminist Life', 'Radiant Textuality', 'The Undercommons']

>>> books[2] # ['Living a Feminist Life']

>>> books[1:4] # ['Cruising Utopia', 'Living a Feminist Life', 'Radiant Textuality']

Evaluation

How would you get Python to print the length of the last book in the list? Hint: this number reflects the length of the string which is the last item in the list. Choose the correct expression from the options below.

  • len(books)
  • print(books[-1])
  • print(len[-1])
  • print(len(books[-1]))*

Keywords

Do you remember the glossary terms from this section?

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