Let's Learn Unit Testing in Python with pytest! 🚀

Unit testing in Python can feel like magic — a little preparation, and you’re ready to squash bugs before they creep into your code. Today, we’re diving into pytest, a powerful yet simple framework that makes testing fun! 😃 Let's learn through examples and keep theory to a minimum. Ready? Let’s go! 🏃

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What is pytest?

pytest is a Python testing framework that’s:

• Simple to use: Write clean tests with minimal boilerplate.
• Feature-rich: Handles fixtures, assertions, parameterized tests, and more.
• Extensible: Add plugins to supercharge your tests.

Here’s how you install it:

pip install pytest

Boom! That’s it. You’re ready. 🚀

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Writing Your First Test 🔬

Here’s the simplest test you can write:

# test_sample.py

def test_addition():
    assert 1 + 1 == 2

To run this test, type:

pytest test_sample.py

You’ll see this output:

Hooray! 🎉 Your test passed.

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The Power of Assertions 🔥

pytest’s magic lies in assertions. You use assert statements to test if your code behaves as expected. Here are some examples:

# test_math.py

def test_operations():
    assert 2 * 2 == 4
    assert 10 / 2 == 5
    assert 5 - 2 == 3

You’ll see this output:

If any assert fails, pytest will show you a detailed error message. 🎉 No need to learn a special syntax!

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Fixtures: Setting the Stage 🎡

Fixtures are a way to set up reusable context for your tests. Imagine you’re testing a database. Instead of connecting to the database in every test, you can create a fixture.

Here’s an example:

import pytest

@pytest.fixture
def sample_data():
    return {"name": "Alice", "age": 30}

def test_sample_data(sample_data):
    assert sample_data["name"] == "Alice"
    assert sample_data["age"] == 30

pytest automatically provides the sample_data fixture to the test function. 🚀

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Parameterized Tests: Test More with Less 🔄

Let’s say you want to test multiple inputs. Instead of writing multiple test functions, you can use @pytest.mark.parametrize:

import pytest

@pytest.mark.parametrize("x, y, result", [
    (1, 2, 3),
    (5, 5, 10),
    (10, -2, 8),
])
def test_add(x, y, result):
    assert x + y == result

pytest will run the test for every combination of inputs! 🔧

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Organizing Your Tests 🗂

Keep your tests organized:

• Test files: Name them test_*.py or *_test.py.
• Test functions: Start with test_.

Example structure:

project/
|-- app.py
|-- tests/
    |-- test_app.py
    |-- test_utils.py

pytest will automatically discover your tests. Neat, right? 😉

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pytest Plugins: Level Up Your Testing 🏆

pytest has tons of plugins to make your life easier. Here are a few favorites:

• pytest-cov: Measure code coverage.

  pip install pytest-cov
  pytest --cov=your_module

• pytest-mock: Mock objects for unit tests.

• pytest-django: For testing Django applications.

Find more plugins at pytest-dev/plugins.

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Handling Expected Errors ⚠️

To test if a function raises an error, use pytest.raises:

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero!")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError, match="Cannot divide by zero!"):
        divide(1, 0)

pytest will check that the error is raised and matches the message. 🔓

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Interactive Quiz 🕺

Question: What does this test output?

def test_example():
    assert 2 * 3 == 5

• A: Test passes ✅
• B: Test fails with an error ❌

Spoiler: It fails! 😅 pytest will output something like:

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Final Tips 🙌

1. Start small: Write simple tests as you learn.
2. Test early: Write tests as you code, not after.
3. Use coverage: Aim for high code coverage but focus on meaningful tests.

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Unit testing with pytest is straightforward, powerful, and fun! 🚀 Start writing tests today and watch your codebase become more robust and reliable. Happy testing! 🎮

Comments

[Can Mingir]

Can you do BDD style like Jasmine in pytest?

[Jagroop Singh]

Yes,I have implement Behavior-Driven Development style tests similar to Jasmine in pytest by using a plugin called pytest-bdd.

[sewiko]

Option B: Test fails with an error ❌ is correct.

[Jagroop Singh]

Yes, it's correct. That's a simple one.

[Anurag Nagar]

I think you can create one post on how to write advanced test cases.

[Jagroop Singh]

Sure

[HB_the_Pencil]

Thanks! This is a really great and in-depth tutorial. I expect to get more use out of it in the near future :D

[Jagroop Singh]

Thanks @hbthepencil

[Draven Locke]

You really make it seem easy and fun especially with how simple the syntax is. I love how you included examples for common scenarios. The tips at the end are really helpful too.

[Jagroop Singh]

thanks @draven_locke_039584510230