Imagine this: you change one tiny line of code in your project, something harmless, you think. You rerun your app, and suddenly nothing works. A feature that worked perfectly yesterday is now broken, and you have no idea why.

Every developer has faced this moment. And this is exactly why testing matters.

Testing is simply making sure your code behaves the way you expect it to. Without tests, every change you make becomes a gamble. You might fix one thing and accidentally break something else.

Automated tests solve this problem. They run your checks for you, instantly and consistently, so you can focus on building instead of constantly rechecking everything.

That's where pytest shines.

pytest is one of the simplest and most beginner-friendly testing tools in Python. It doesn't require complicated setup or special classes; you just write normal Python functions and use plain assert statements.

If you've been putting off learning testing, this is the perfect place to start.

What Is pytest?

pytest is a tool that helps you write small, simple tests in Python. Instead of relying on manual checks or print statements, pytest lets you create tiny test functions that verify your code automatically.

Here's why beginners love pytest:

  • Minimal setup: Install it once, and you're ready to start testing
  • Easy-to-read tests: Tests look like normal Python functions
  • Uses plain assert statements: Just write assert something == expected
  • Automatically finds your tests: Name your file test_something.py, and pytest discovers it

Installing pytest

Installing pytest is straightforward:

pip install pytest

If you're working in a virtual environment (recommended), make sure it's activated first.

Verify the installation:

pytest --version

You should see something like pytest 9.02. Congratulations, pytest is installed and ready to use!

Writing Your First Test

Let's write your very first test. It's simpler than you might think.

Step 1: Create a simple function

Create a file called calculator.py:

def add_numbers(a, b):
    """Add two numbers together."""
    return a + b

Step 2: Write your first test

Create a test file called test_calculator.py:

from calculator import add_numbers

def test_add_numbers():
    result = add_numbers(2, 3)
    assert result == 5

That's it! This is a complete pytest test:

  • def test_add_numbers(): - The test_ prefix tells pytest this is a test function
  • result = add_numbers(2, 3) - We call our function with test inputs
  • assert result == 5 - If the result equals 5, the test passes

Step 3: Run pytest

Open your terminal and run:

pytest

Pytest will automatically find your test file and run it. You should see:

================================================== test session starts ===================================================
platform win32 -- Python 3.12.0, pytest-9.0.2, pluggy-1.6.0
rootdir: D:\GitHub\pytest-article
collected 1 item                                                                                                          

test_calculator.py .                                                                                                [100%]

=================================================== 1 passed in 0.04s ====================================================

The dot (.) means your test passed!

What if the test fails?

If a test fails, pytest shows you exactly what went wrong:

================================================== test session starts ===================================================
platform win32 -- Python 3.12.0, pytest-9.0.2, pluggy-1.6.0
rootdir: D:\GitHub\pytest-article
collected 1 item                                                                                                          

test_calculator.py F                                                                                                [100%]

======================================================== FAILURES ======================================================== 
_________________________________________________ test_add_numbers_wrong _________________________________________________ 

    def test_add_numbers_wrong():
        result = add_numbers(2, 3)
>       assert result == 10  # This is wrong!
        ^^^^^^^^^^^^^^^^^^^
E       assert 5 == 10

test_calculator.py:9: AssertionError
================================================ short test summary info =================================================
FAILED test_calculator.py::test_add_numbers_wrong - assert 5 == 10
=================================================== 1 failed in 0.47s ====================================================

The F means failed, and the error message shows you expected 10 but got 5. Pytest's clear error messages help you fix issues quickly.

Full source code at:

GitHub - nunombispo/pytest-article
Contribute to nunombispo/pytest-article development by creating an account on GitHub.
GitHubnunombispo

Want to boost your Python skills even faster?

Before you dive deeper into testing, grab my free Python One-Liner Cheat Sheet, a downloadable PDF packed with smart, time-saving tricks for real-world coding. You’ll learn elegant list & dict comprehensions, clever string shortcuts, functional patterns in a single line, quick file-handling snippets, and pro tips to keep your code clean and Pythonic.

How pytest Discovers Tests

Pytest automatically finds your tests using simple naming rules:

  1. Test files must start with test_ or end with _test.py
  2. Test functions must start with test_

Valid examples:

  • test_calculator.py
  • calculator_test.py
  • def test_add_numbers():

Invalid examples:

  • calculator.py
  • def add_numbers_test():

You can organize tests in a tests folder or keep them next to your code, both work fine. Just follow the naming rules, and pytest will find them automatically.

Understanding pytest Output

Pytest uses simple symbols to show test results:

  • . (dot) = Test passed
  • F = Test failed
  • s = Test skipped

When you see test_calculator.py ..F, it means:

  • First test passed (.)
  • Second test passed (.)
  • Third test failed (F)

When a test fails, pytest shows:

  • The > arrow pointing to the exact line that failed
  • The error message (e.g., assert 5 == 10)
  • The file and line number where it happened

Common errors:

  • AssertionError: assert X == Y - Your assertion was wrong
  • NameError: name 'X' is not defined - Check your imports
  • TypeError: unsupported operand type(s) - Check your data types

Using Fixtures

As you write more tests, you'll notice many tests need the same setup. Instead of repeating code, use fixtures, reusable setup for your tests.

Example: