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.

• 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’.

So 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:

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
   if add(1, 2) != 3:
      print("Test failed!")
      raise AssertionError("Test failed!")

   # Check that it adds zero
   if add(5, 0) != 5:
      print("Test failed!")
      raise AssertionError("Test failed!")

   # Check that it adds two negative integers
   if add(-1, -2) != -3:
      print("Test failed!")
      raise AssertionError("Test failed!")

(Note that the AssertionError is a way to tell Python to crash the test, so Pytest knows 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 functions with the prefix test_.

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!

The assert keyword

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Writing these if blocks for each test case is cumbersome. Fortunately, Python has a keyword to do this for us - the assert keyword.

The assert keyword checks if a statement is true and if it is, the test continues, but if it isn’t, then the test will crash, printing an error in the terminal. This enables us to write succinct tests without lots of if-statements.

The assert keyword is used like this:

PYTHON

assert add(1, 2) == 3

which is equivalent to:

PYTHON

if add(1, 2) != 3:
  # Crash the test
  raise AssertionError

Challenge 1: Use the assert keyword to update the test for the add function

Use the assert keyword to update the test for the add function to make it more concise and readable.

Then re-run the test using pytest ./ to check that it still passes.

Show me the solution

PYTHON

from calculator import add

def test_add():
  assert add(1, 2) == 3 # Check that it adds to positive integers
  assert add(5, 0) == 5 # Check that it adds zero
  assert add(-1, -2) == -3 # Check that it adds wro negative numbers

Now that we are using the assert keyword, pytest will let us know if our test fails.

What’s more, is that if any of these assert statements fail, it will flag to pytest that the test has failed, and pytest will let you know.

Make the add function return the wrong value, and run the test again to see that the test fails and the text turns red as we expect.

So 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 2: 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

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

Run the test using pytest ./ to check that it passes. If it doesn’t, don’t worry, that’s the point of testing - to find bugs in code.

Key Points

• The assert keyword is used to check if a statement is true and is a shorthand for writing if statements in tests.
• 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.