Single responsibility per test in PyTest

Introduction

Each test should check only one thing. This makes tests easier to understand and fix.

When you want to find exactly what is broken quickly.

When tests become hard to read because they check many things.

When you want to keep tests simple and clear.

When you want to avoid one test failing for many reasons.

When you want to write tests that are easy to maintain.

Syntax

PyTest

def test_function():
    # Arrange
    # Act
    # Assert
    assert something == expected_value

Each test function should focus on one behavior or feature.

Use clear names to show what the test checks.

Examples

This test checks adding two positive numbers only.

PyTest

def test_add_positive_numbers():
    result = add(2, 3)
    assert result == 5

This test checks adding two negative numbers only.

PyTest

def test_add_negative_numbers():
    result = add(-1, -4)
    assert result == -5

This test checks adding zero to a number only.

PyTest

def test_add_zero():
    result = add(0, 5)
    assert result == 5

Sample Program

This example shows three tests. Each test checks one simple case of the add function. This way, if one test fails, you know exactly which case has a problem.

PyTest

def add(a, b):
    return a + b


def test_add_positive_numbers():
    result = add(2, 3)
    assert result == 5

def test_add_negative_numbers():
    result = add(-1, -4)
    assert result == -5

def test_add_zero():
    result = add(0, 5)
    assert result == 5

Important Notes

Keep tests small and focused to find bugs faster.

One test = one reason to fail.

Good test names help understand what is tested.

Summary

Write tests that check only one thing.

This makes tests easier to read and fix.

Clear test names help everyone understand the test purpose.