Parameterized Testing

Estimated time: 25 minutes

Overview

Questions

• What is a test framework?
• How does a test framework like pytest make writing test cases easier?

Objectives

• Start writing cleaner tests with pytest.

The Problem of Test Duplication

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Now that you have a passing test suite, look closely at your test_overlap.py file. If you wrote three or four different test cases, your file probably looks something like this:

PYTHON

from overlap import check_overlap

def test_overlapping_rectangles():
    rect_a = (0, 0, 2, 2)
    rect_b = (1, 1, 3, 3)
    assert check_overlap(rect_a, rect_b) is True

def test_separated_rectangles():
    rect_a = (0, 0, 2, 2)
    rect_b = (3, 3, 5, 5)
    assert check_overlap(rect_a, rect_b) is False

def test_subset_rectangles():
    rect_a = (0, 0, 4, 4)
    rect_b = (1, 1, 2, 2)
    assert check_overlap(rect_a, rect_b) is True

This works, but it is highly repetitive. Every single one of these functions does the exact same thing: it sets up two coordinate tuples, feeds them into check_overlap, and asserts a boolean result.

If you want to test 50 different coordinate combinations, you do not want to copy-paste this boilerplate 50 times. If you change your function’s signature later, you will have to rewrite 50 different functions!

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The Pytest Way: @pytest.mark.parametrize

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Pytest solves this problem using a decorator called parametrize. This allows you to write the test logic exactly once and pass in a list of different inputs and expected outputs.

Here is how it works:

PYTHON

import pytest
from overlap import check_overlap

@pytest.mark.parametrize(
    "rect_a, rect_b, expected",
    [
        ((0, 0, 2, 2), (1, 1, 3, 3), True),   # Overlapping
        ((0, 0, 2, 2), (3, 3, 5, 5), False),  # Separated
    ]
)
def test_overlap_scenarios(rect_a, rect_b, expected):
    assert check_overlap(rect_a, rect_b) is expected

What is Happening Here?

1. The Decorator: @pytest.mark.parametrize tells pytest that this function is a template.
2. The Argument Names: The first string "rect_a, rect_b, expected" defines the variable names that will be passed into our test function.
3. The Data Matrix: The list of tuples contains our actual test cases. Each tuple in this list represents a single run of the test.
4. The Test Runner: When you run pytest, it treats each item in the list as an entirely separate test case. If the second case fails, the first one still passes!

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Discussion

Exercise: Parameterize Your Overlap Tests (10 mins)

1. Open test_overlap.py.
2. Refactor your individual test functions into a single, clean parameterized test function called test_overlap_scenarios.
3. Add at least five different scenarios to your parametrization list (including at least one edge case, like completely identical rectangles or one rectangle entirely inside another).
4. Run pytest -v (the -v flag stands for “verbose”) in your terminal. Observe how pytest dynamically generates names for each of your parameterized runs!

Show me the solution

Here is a clean, robust way to parameterize your overlap tests using coordinate tuples:

PYTHON

import pytest
from overlap import check_overlap

@pytest.mark.parametrize(
    "rect_a, rect_b, expected",
    [
        ((0, 0, 2, 2), (1, 1, 3, 3), True),    # Scenario 1: Partial overlap
        ((0, 0, 2, 2), (3, 3, 5, 5), False),   # Scenario 2: Distant separation
        ((0, 0, 4, 4), (1, 1, 2, 2), True),    # Scenario 3: B inside A (nested)
        ((0, 0, 2, 2), (0, 0, 2, 2), True),    # Scenario 4: Identical coordinates
        ((0, 0, 2, 2), (2, 0, 4, 2), False),   # Scenario 5: Shared boundary (touching edge)
    ]
)
def test_overlap_scenarios(rect_a, rect_b, expected):
    assert check_overlap(rect_a, rect_b) is expected

If you run this with pytest -v, your terminal output will look like this, showing each run as its own independent check:

BASH

test_overlap.py::test_overlap_scenarios[rect_a0-rect_b0-True] PASSED
test_overlap.py::test_overlap_scenarios[rect_a1-rect_b1-False] PASSED
test_overlap.py::test_overlap_scenarios[rect_a2-rect_b2-True] PASSED
test_overlap.py::test_overlap_scenarios[rect_a3-rect_b3-True] PASSED
test_overlap.py::test_overlap_scenarios[rect_a4-rect_b4-False] PASSED

Key Points

• A test framework simplifies adding tests to your project.
• Choose a framework that doesn’t get in your way and makes testing fun.
• Coverage tools are useful to identify which parts of the code are executed with tests