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Module 4: Python Lists

WELCOME

In this module, we introduce Python’s list, the most commonly used data
structure that is built into the language itself. Just like any list of stuff
you encounter in everyday life, a list in Python is just a bunch of items
arranged in some order. You can add items to a list and also remove items from
it. What can the items be? Anything!

Why are lists so useful? Let’s recall what computers do so well - repetitive tasks. So a list basically contains a potentially enormous amount of little tasks (or data), and the computer simply goes through the items one after the next, very very quickly. The possibilities are endless!

Lesson

Topics

  • List creation
  • Important list methods
  • Indexing lists
  • Indexing strings
  • The range function

Introduction to Lists

So far, we’ve covered 3 main data types in Python: strings, integers, and booleans. Next, we’ll cover lists in Python. At it’s most basic level, a Python list is meant to keep a collection of other values in order. Here’s an example:

desserts = ['cupcake', 'ice cream', 'cookie']

In the above code, the variable desserts has the value of a Python list. While there are other data types contained in the list (strings, in this case), it’s important to recognize that the value of the variable desserts as a whole is a list.

Syntax

There are a couple of important things to note about the way we can define lists in Python.

  1. A list begins and ends with a square bracket character, ie [ and ].
  2. The items in a list are separated by commas.

Finding the length of a list

It’s possible to get the length of the list using the same built-in len function that we’ve used to get the length of a string:

>>> desserts = ['cupcake', 'ice cream', 'cookie']
>>> print len(desserts)
3

We can also store that value of 3 in a variable, like so:

>>> desserts = ['cupcake', 'ice cream', 'cookie']
>>> desserts_length = len(desserts)
>>> print dessers_length

Empty Lists

It’s possible make a list with no items.

desserts = []

What’s the length of an empty list?

An empty list’s length is zero

>>> desserts = []
>>> print len(desserts)
0

Adding Items to a List

It’s possible add a items to our empty desserts list using the method called append.

desserts = []
desserts.append('cupcake')

What does our desserts list look like now? How could we check?

Let’s print the desserts list

>>> print desserts
['cupcake']

So, a one-item list is great, but we’d like to add a couple more items, like so:

desserts.append('ice cream')
desserts.append('cookie')

What do you think the list looks like now?

Option A) ['cupcake, ice cream, cookie']

Option B) ['cupcake', 'ice cream', 'cookie']

Option C) ['cookie']

Option D) ['cookie', 'ice cream', 'cupcake']

Option B

The list has 3 separate strings in it. As each item was added, or more specifically appended, to the list, it was put at the end of the list.

Accessing items in a list

We can ask for a particular item in a list, or index the list, using the following syntax:

>>> desserts[0]
'cupcake'
>>> desserts[1]
'ice cream'
>>> desserts[2]
'cookie'

The place of an item in a list, with regards to the order of the list, is called the item’s index. In the desserts list, the string 'cupcake' has the index of 0, the string 'ice cream' has the index of 1, and the string 'cookie' has the index of 2.

Interestingly, the first item in the list does not have the index of 1. Python list indexing starts at zero, rather than one. This can be a little tricky at first, so let’s do some practice.

Check for Understanding: List Indexing

Given the following list:

languages = ['Portuguese', 'English', 'Spanish', 'Russian', 'Mandarin']

How would you do the following?

Try it out!

Try this out in a repl session console by copying the languages list, and attempting to index the list.

  • Index the list in order to print out Spanish?

print languages[2]

The third item in languages has the index of 2.

  • Index the list in order to print out Portuguese?

print languages[0]

The first item in languages has the index of 0.

  • Index the list in order to print out Mandarin?

print languages[4]

The fifth item in languages has the index of 4.

Index Errors

If you try to access an item at an index that does not exist in a list, you get an IndexError. For example, given the following two-item list, there is item at index 0 and another item at index 1.

colors = ['purple', 'pink']

We can safely ask for both the item at index 0 and the item at index 1:

>>> print colors[0]
'purple'
>>> print colors[1]
'pink'

However, if we try to get the item at index 2, there’s an error.

>>> print colors[2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

As we’ll see in the control flow module, this is a fairly common error to get. However, its also a very easy error to fix. Getting an IndexError simply means you’re trying to get something that isn’t there.

Two More List Methods

Just like strings, lists have a variety of methods, or programmatic behaviors or capabilities. Methods are always notated like this:

some_data_type.method_name()

The append method is perhaps the most common and important method for lists in Python.

Let’s learn about 2 more methods.

The pop list method

There are several ways to remove items from a list. In order to remove the last item in a list, you can use the method pop.

Here is an example:

>>> desserts = ['cupcake', 'ice cream', 'cookie']
>>> desserts.pop()
'cookie'
>>> print desserts
['cupcake', 'ice cream']

After pop ran, 'cookie' was removed!

Try this out for yourself in the Python repl console.

The sort list method

One of the wonderful thing about lists in Python is that they keep track of order. Once the list is defined, the order of the items is maintained unless purposefully modified. So, the desserts list will always have "cupcake" first, and "ice cream" second, and so on.

Let’s learn how to change the order! It’s conceivable that we would want a list of strings in alphabetical order, rather than the original order:

>>> desserts = ['cupcake', 'ice cream', 'cookie']
>>> desserts.sort()
>>> print desserts
['cookie', 'cupcake', 'ice cream']

The list is now in alphabetical order. Nice!

What goes in a list?

Up until now, we’ve been storing strings in our lists. It’s also possible to make lists of integers and booleans as well. Here are a couple of examples:

>>> my_numbers = [ 2, 4, 6, 8, 10 ]
>>> some_booleans = [ True, True, False, True, True ]

What would my_numbers[0] give you?

It’s the first item!

>>> print my_numbers[0]
2
>>> type(my_numbers[0])
<type 'int'>

What would some_booleans[2] give you?

It’s the third item!

>>> print some_booleans[2]
False
>>> type(some_booleans[2])
<type 'bool'>

Heterogenenous Lists

Additionally, you can store many different data types in the same list. Here’s an example:

>>> stuff = ['cupcake', True, 47]

What would stuff[1] give you?

It’s the second item!

>>> print stuff[1]
True
>>> type(stuff[1])
<type 'bool'>

String Indexing

While strings and lists are different in many ways, they also share some of the same characteristics. Just like a list, it’s also possible to index a string.

For example, we let’s say we have a variable called my_name and it’s value is the string "Balloonicorn":

>>> my_name = "Balloonicorn"
>>> something = my_name[0]

What will happen when the following code runs?:

>>> print something

The first letter of "Balloonicorn" gets printed

Using square brackets to index a string results in obtaining one of its characters. This is pretty nifty!

>>> my_name = "Balloonicorn"
>>> something = my_name[0]
>>> print something
B
>>> something = my_name[0]

What will happen when the following code runs?:

>>> my_pet = "Arfy"
>>> print my_pet[2]

The third letter of "Arfy" gets printed

Since strings are also zero-indexed, asking for the letter at index 2 gives us the letter f.

>>> my_pet = "Arfy"
>>> print my_pet[2]
f

At index 0 is the letter A, at index 1 is the letter r, and at index 2 is the letter f.

The range Function

Now that we’ve learned lists in Python, it’s time to learn about a very common built-in function in Python that produces a list.

First, lets review other built-in functions we know. Which built-in functions have we learned about?

type and len

We’ve learned about 2 built-in functions so far. A built-in function is a function that can be called in any Python code, without having to define it or import it from somewhere else.

As a review, here’s how we’ve used type and len:

>>> my_message = "Hello world"
>>> type(my_message)
<type 'str'>
>>> len(my_message)
11

type shows the data type for whatever is passed in to the type function. In this case, we passed a variable, my_message, whose value is a string.

len shows the length of whatever is passed in. In this case, we passed in a string which has 11 characters (including the space).

A third built-in function is called range. You must pass it in an integer as an argument, and it will return a list of integers.

For example:

>>> range(3)
[0, 1, 2]

In this first example, we passed in 3 to the range function. So, range returned a list of 3 numbers. The numbers begin at 0, just like indices in lists and strings.

Here are a couple more examples:

>>> my_nums = range(4)
>>> print my_nums
[0, 1, 2, 3]
>>> more_nums = range(18)
>>> print more_nums
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]
>>> more_nums = range(5)
>>> print more_nums
[0, 1, 2, 3, 4]

Practice Section

Directions

Login to your account and start a new repl here. Title it module_4_practice.py.

Complete the practice problems below on Repl.it.

  1. Make a variable characters, whose value is a list. Inside the list should be at least 5 names of your favorite book or TV characters, as strings.
  2. Make a variable furniture, whose value is a list. Inside the list should be at least 4 names of pieces of furniture, as strings.
  3. Make a variable odd_nums, whose value is a list. Inside the list should be 5 odd numbers, as integers.
  4. Make a variable even_nums, whose value is a list. Inside the list should be 5 even numbers, as integers.
  5. Make a variable todo_list, whose value is a list. Inside the list should be two things you need to accomplish today, as strings.
  6. Append one character to your list called characters.
  7. Append one piece of furniture to your list called furniture.
  8. Append one odd integer to your list called odd_nums.
  9. Append one even integer to your list called even_nums.
  10. Append one To Do List item to your list called todo_list.
  11. Make a list of the following numbers in the following order: 0, 1, 2, 3, 4, 5, 6. Hover over the section below if you’d like a hint.

Use the range built-in function

The built-in function called range is very good at making lists of numbers. Use it!

  1. Remove the last item from your todo_list variable using the list method called pop.
  2. Remove the last item from your furniture variable using the list method called pop.
  3. Using list indexing, print out the third odd number in your list called odd_nums. Remember, indices start at 0, not 1. So, what index does the third item have? Hover below to reveal.

The third item has the index of 2

Since the first item in your odd_nums list has the index of 0, the second has the index of 1, and the third has the index of 2.

  1. Using list indexing, print out the fifth even number in your list called even_nums. Remember, indices start at 0, not 1. So, what index does the fifth item have? Hover below to reveal.

The fifth item has the index of 4

Since the first item in your odd_nums list has the index of 0, the second has the index of 1, and so on, the fifth item has the index of 4.

  1. Using the following code, print the fifth letter in the variable called word1– the letter c. Do this using string indexing:
>>> word1 = "delicious"
  1. Using the following code, print the second letter in the variable called word2– the letter u. Do this using string indexing:
>>> word2 = "yummy"
  1. Given the following list of numbers that are not in order, sort the list. You can do this using the list method called sort:
>>> some_numbers = [4, 2, 1, 6, 8]

Debugging

Directions

As a programmer, debugging is a fact of life. There are times you write code that Python doesn’t understand. In these cases, Python will display an error message. The more familiar you are with Python’s many error messages, the faster you’ll be at debugging code. But there’s good news: Python’s error messages are incredibly descriptive and helpful in figuring out what the problem is.

In the following problems, you’ll find code that is invalid or not allowed in some way. Read the code, and see if you can predict what is wrong. When you’re ready, hover over the solution area to reveal the error message that Python shows, along with an explanation of what is going wrong.

1) What’s wrong with this code?

>>> pizza_toppings = ['pepperoni', 'olives', 'mushrooms']
>>> print pizza_toppings[3]

IndexError since there is no item at index 3

In order for a list to have something at the index of 3, there would need to be 4 items .

Item Place Index
first item 0
second item 1
third item 2

So, this code throws in IndexError.

>>> pizza_toppings = ['pepperoni', 'olives', 'mushrooms']
>>> print pizza_toppings[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
  1. What’s wrong with this code?
>>> desserts = []
>>> desserts.add('cupcake')

Must use append to add an item to end of list

The list method to use in order to add something to the end of a list is called append. Lists don’t have a method called add.

So, this code throws in AttributeError.

>>> desserts = []
>>> desserts.add('cupcake')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'add'
  1. What’s wrong with this code?
>>> desserts = []
>>> print desserts[0]

IndexError since there is no item at index 0

In order for a list to have something at the index of 0, there would need to be at least one item. An empty list has nothing at index 0.

So, this code throws in IndexError.

>>> desserts = []
>>> print desserts[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

External Resources

Videos

Socratica Python Lists

List Methods

  • Note: This video goes a little beyond what this module covers, but it’s a good video for those who’d like to go a bit further.

Final Assignment

Create a new repl session called module_4_printing.py.

Part 1

Ask the user as series of questions using raw_input, capturing their input into appropriately-named variables. The questions should be

  • Where would you like to travel to? Name one place.
  • What is another place you’d like to travel to? Name one place.
  • What is one more place you’d like to travel to? Name one place.

Add each piece of user input to an empty list that you create. Please make sure to give the list an appropriate name.

Sort the list of places.

Then, tell the user where they said they’d like to go. Show each place on a separate line.

Part 2

Copy the following list into your file:

>>> fun_words = ["elephant", "balloon", "macchiato", "angostura"]

Then, make a two empty lists– one called first_letters and one called third letters.

Using list_indexing along with string_indexing, make a variable for the first letter of each word in the fun_words list.

Need a hint?

Double index

If you index the fun_words list to get out a word, and then index the word to get out a letter, you’re halfway there.

first_word = fun_words[0]
first_letter_first_word = first_word[0]

All that’s left is to put that letter into your first words list.

Part 3

Copy the following 3 lists into your file:

>>> websites = ["facebook", "twitter", "buzzfeed"]
>>> fruits = ["apple", "banana", "mango", "berry"]
>>> names = ["Bob", "Alice", "Henry", "Rick", "Carl"]

Find the length of each of these lists, and store this value in three separate variables.

Make an empty list called lengths.

Add each length to the lengths lists.

Part 4

Make a list of 25 numbers. Do not hard-code the numbers– that is, don’t type out 25 numbers yourself. Review the lesson if you’re not sure how to make a list of numbers using Python.

Once you have your list of 25 numbers, print out the first, fifth, tenth, and fifteenth item in your list.