Simple Tests

Overview

Questions

• How to write a simple test?
• How to run the test?

Objectives

• Write a basic test.
• Run the test.
• Understand its output in the terminal.

Your first test

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The most basic thing you will want to do in a test is check that an output for a function is correct by checking that it is equal to a certain value.

Let’s take the add function example from the previous chapter and the test we conceptualised for it and write it in code. We’ll aim to write the test in such a way that it can be run using Pytest, the most commonly used testing framework in Python.

• Make a folder called my_project (or whatever you want to call it for these lessons) and inside it, create a file called ‘calculator.py’, and another file called ‘test_calculator.py’.

Your directory structure should look like this:

BASH

project_directory/
│
├── calculator.py
└── test_calculator.py

calculator.py will contain our Python functions that we want to test, and test_calculator.py will contain our tests for those functions.

• In calculator.py, write the add function:

PYTHON

def add(a, b):
    return a + b

• And in test_calculator.py, write the test for the add function that we conceptualised in the previous lesson, but use the assert keyword in place of if statements and print functions:

PYTHON

# Import the add function so the test can use it
from calculator import add

def test_add():
    # Check that it adds two positive integers
    assert add(1, 2) == 3

    # Check that it adds zero
    assert add(5, 0) == 5

    # Check that it adds two negative integers
    assert add(-1, -2) == -3

The assert statement will crash the test by raising an AssertionError if the condition following it is false. Pytest uses these to tell that the test has failed.

This system of placing functions in a file and then tests for those functions in another file is a common pattern in software development. It allows you to keep your code organised and separate your tests from your actual code.

With Pytest, the expectation is to name your test files and functions with the prefix test_. If you do so, Pytest will automatically find and execute each test function.

Now, let’s run the test. We can do this by running the following command in the terminal:

(make sure you’re in the my_project directory before running this command)

BASH

❯ pytest

This command tells Pytest to run all the tests in the current directory.

When you run the test, you should see that the test runs successfully, indicated by some green. text in the terminal. We will go through the output and what it means in the next lesson, but for now, know that green means that the test passed, and red means that the test failed.

Try changing the add function to return the wrong value, and run the test again to see that the test now fails and the text turns red - neat! If this was a real testing situation, we would know to investigate the add function to see why it’s not behaving as expected.

Challenge

Write a test for a multiply function

Try using what we have covered to write a test for a multiply function that multiplies two numbers together.

• Place this multiply function in calculator.py:

PYTHON

def multiply(a, b):
    return a * b

• Then write a test for this function in test_calculator.py. Remember to import the multiply function from calculator.py at the top of the file like this:

PYTHON

from calculator import multiply

Show me the solution

There are many different test cases that you could include, but it’s important to check that different types of cases are covered. A test for this function could look like this:

PYTHON

def test_multiply():
    # Check that positive numbers work
    assert multiply(5, 5) == 25
    # Check that multiplying by 1 works
    assert multiply(1, 5) == 5
    # Check that multiplying by 0 works
    assert multiply(0, 3) == 0
    # Check that negative numbers work
    assert multiply(-5, 2) == -10

Key Points

• The assert keyword is used to check if a statement is true.
• Pytest is invoked by running the command pytest ./ in the terminal.
• pytest will run all the tests in the current directory, found by looking for files that start with test_.
• The output of a test is displayed in the terminal, with green text indicating a successful test and red text indicating a failed test.
• It’s best practice to write tests in a separate file from the code they are testing. Eg: scripts.py and test_scripts.py.