Lists

You can find all the code for this chapter here

Lists in Python are a collection of items that can be of different types. They are mutable, meaning that you can change the elements in a list after you have created it.

It’s possible to operate them in different ways, like adding, removing, or checking if an element is in the list.

The access is made by an index, which is an integer that represents the position of the element in the list. The index starts at 0.

Add an item to a list

Enough talk, let’s implement the add operation.

Write the test first

As always, we start by writing a test that describes the behavior we want to implement.

from expects import expect, equal


class TestList:
    def test_add(self):
        list = add([], 1)

        expect(list).to(equal([1]))

Try and run the test

If we run the test, we should see an error.

================= short test summary info ================= 
FAILED test_list.py::TestList::test_list
NameError: name 'add' is not defined
================= 1 failed in 0.02s =======================

Write the minimal amount of code to make it run

Let’s implement the add function with the enough code to make the test compile.

def add(list, item):
    pass

If we run the test again we should see a different error.

E       expected: None to be [1]
================= short test summary info ================= 
FAILED test_list.py::TestList::test_list
AssertionError: 
================= 1 failed in 0.02s ============

Write enough code to make it pass

Now it’s time to write the code that makes the test pass.

def add(list, item):
    return [item]

If we run the test again it should pass.

========================= 1 passed in 0.01s ========================= 

Refactor

Let’s add more items in the list, which will force us to refactor the code.

from expects import expect, equal


class TestList:
    def test_add(self):
        list = add([], 1)
        list = add(list, 2)

        expect(list).to(equal([1, 2]))

Finally, we can refactor the code to make it more general.

def add(list, item):
    return list + [item]

Remove an item from a list

What about removing an item from a list? Let’s give it a try!

Write the test first

As always, we start by writing a test that describes the behavior we want to implement.

from expects import expect, equal


class TestList:
    def test_remove(self):
        list = [1, 2, 3]
        list = remove(list, 1)

        expect(list).to(equal([2, 3]))

Try and run the test

If we run the test, we should see an error.

================= short test summary info ================= 
FAILED test_list.py::TestList::test_remove
NameError: name 'remove' is not defined
================= 1 failed in 0.02s =======================

Write the minimal amount of code to make it run

Let’s implement the remove function with the enough code to make the test compile.

def remove(list, item):
    pass

If we run the test again we should see a different error.

E       expected: None to be [2, 3]
================= short test summary info ================= 
FAILED test_list.py::TestList::test_remove
AssertionError: 
================= 1 failed in 0.02s ============

Write enough code to make it pass

Now it’s time to write the code that makes the test pass.

def remove(list, item):
    return [2 ,3]

If we run the test again it should pass.

========================= 1 passed in 0.01s ========================= 

Refactor

Let’s remove more items in the list, which will force us to refactor the code.

from expects import expect, equal
from list import remove


class TestList:
    def test_add(self):
        list = [1, 2 ,3]

        list = remove(list, 2)
        list = remove(list, 3)

        expect(list).to(equal([1]))

Finally, we can refactor the code to make it more general.

def remove(list, item):
    new_list = []
    for i in list:
        if i != item:
            new_list = add(new_list, i)
    return new_list

Note: We’re using a for loop, don’t worry about it we’ll talk about it later in this book.

Wrapping up

What we have covered:

• More practice of the TDD workflow.
• Working with lists, adding and removing items.

Your turn

A great way to learn is doing, so here are some ideas to keep practicing:

• Write a test to calculate the length of a list.
• Write a test to check if a list contains an item.
• Write a test to compare two lists.