Introduction to fixtures

Introduction to Testing in Python

Alexander Levin

Data Scientist

What is a fixture

Fixture - a prepared environment that can be used for a test execution
Fixture Setup - a process of preparing the environment and setting up resources that are required by one or more tests

Imagine preparation for a picnic:

Invite our friends and prepare the food (that's what fixtures do)
Have fun!
Clean up

Why do we need fixture

Fixtures help:

To make test setup easier
To isolate the test of the environmental preparation
To make the fixture code reusable

Fixture example: overview

Assume we have:

a Python list variable named data
data = [0, 1, 1, 2, 3, 5, 8, 13, 21]

And we want to test, that:

It contains 9 elements
It contains the elements 5 and 21

Fixture example: code

import pytest

# Fixture decorator
@pytest.fixture
# Fixture for data initialization
def data():
    return [0, 1, 1, 2, 3, 5, 8, 13, 21]

def test_list(data):
    assert len(data) == 9
    assert 5 in data
    assert 21 in data

Fixture example: output

Output of the example: output of the fixture example

How to use fixtures

To use the fixture we have to do the following:

Prepare software and tests
Find "environment preparation"
Create a fixture:
- Declare the @pytest.fixture decorator
- Implement the fixture function
Use the created fixture:
- Pass the fixture name to the test function
- Run the tests!

Summary

We learned about testing fixtures:

Fixture - a prepared environment that can be used for a test execution
We use fixtures to make test setup easier and isolated from the test functions
Simple example: preparation of a Python list
Define a pytest fixture by declaring @pytest.fixture
- followed by a fixture function
Fixture names are used in the tests as variables

Let's practice!

Introduction to Testing in Python