Using Python Lists

Note: The using-lists.py file exists in the /home/cloud_user/ directory. If you do not see the file, please give the lab another minute to finish provisioning. If you prefer to use your own local Python editor, you can also access the initial using-lists.py file in the lab's GitHub repository.

Our first few tasks require us to create the users variable that we're going to work with throughout the lab and then add some information to it. Here's how we complete the first task:

using-lists.py (partial)

# 1) Set the users variable to be an empty list
users = []

assert users == [], f"Expected `users` to be [] but got: {repr(users)}"

Now if we run the file we should see the error for the second task:

$ python3.7 using-lists.py
Traceback (most recent call last):
  File "./using-lists.py", line 8, in <module>
    assert users == ['kevin', 'bob', 'alice'], f"Expected `users` to be ['kevin', 'bob', 'alice'] but got: {repr(users)}"
AssertionError: Expected `users` to be ['kevin', 'bob', 'alice'] but got: []

This error shows us that we need to add values to the list before we can continue. The task also specifies that we shouldn't just reassign the users variable. Here's an example solution to this:

using-lists.py (partial)

# 2) Add 'kevin', 'bob', and 'alice' to the users list in that order without reassigning the variable.
users.append('kevin')
users.append('bob')
users.append('alice')

assert users == ['kevin', 'bob', 'alice'], f"Expected `users` to be ['kevin', 'bob', 'alice'] but got: {repr(users)}"

For tasks 3 and 4 we need to remove the bob value and then create a new reversed version of the new list. Here's an example solution for both of these tasks:

using-lists.py (partial)

# 3) Remove 'bob' from the `users` list without reassigning the variable.
del users[1]

assert users == ['kevin', 'alice'], f"Expected `users` to be ['kevin', 'alice'] but got: {repr(users)}"

# 4) Reverse the users list and assign the result to `rev_users`
rev_users = list(reversed(users))

assert rev_users == ['alice', 'kevin'], f"Expected `rev_users` to be ['alice', 'kevin'] but got: {repr(rev_users)}"

The del statement will allow us to remove an item at a specific index to complete task three. For task four, we'll use the reversed function and then cast the result of that back to a list using the list function.

For tasks five and six we'll be adding more information to our users list. To add a single item at the first index we'll use the insert method. To add three new items to the end of the list we'll use list concatenation and reassign the users variable.

using-lists.py (partial)

# 5) Add the user 'melody' to users where 'bob' used to be.
users.insert(1, 'melody')

assert users == ['kevin', 'melody', 'alice'], f"Expected `users` to be ['kevin', 'melody', 'alice'] but got: {repr(users)}"

# 6) Add the users 'andy', 'wanda', and 'jim' to the users list using a single command
users += ['andy', 'wanda', 'jim']

assert users == ['kevin', 'melody', 'alice', 'andy', 'wanda', 'jim'], f"Expected `users` to be ['kevin', 'melody', 'alice', 'andy', 'wanda', 'jim'] but got: {repr(users)}"

For the final task, we'll extract a subsection from the users list to create the new center_users list. We can extract the third and fourth items from the list by slicing from 2 to 4.

using-lists.py (partial)

# 7) Slice the users lists to return the 3rd and 4th items and assign the result to `center_users`
center_users = users[2:4]

assert center_users == ['alice', 'andy'], f"Expected `users` to be ['alice', 'andy'] but got: {repr(center_users)}"