Assert with messages in PyTest

Introduction

Assertions check if something is true in tests. Adding messages helps explain what went wrong if the test fails.

When you want to know why a test failed quickly.

When multiple checks are in one test and you need clear failure reasons.

When sharing tests with others who need clear feedback.

When debugging tests to save time understanding errors.

Syntax

PyTest

assert condition, "message"

The message shows only if the assertion fails.

Use clear, simple messages to explain the failure.

Examples

This assertion will pass silently because 2 + 2 equals 4.

PyTest

assert 2 + 2 == 4, "Math is broken!"

This checks if 'a' is in 'cat'. It passes silently.

PyTest

assert 'a' in 'cat', "Letter 'a' not found in word"

This will fail and show the message "Five is not greater than ten".

PyTest

assert 5 > 10, "Five is not greater than ten"

Sample Program

This test checks three conditions about the variable number. The first two pass, but the last fails and shows the message.

PyTest

def test_number():
    number = 10
    assert number > 5, "Number should be greater than 5"
    assert number < 20, "Number should be less than 20"
    assert number != 10, "Number should not be exactly 10"

Important Notes

Assertion messages help quickly find the problem without guessing.

Keep messages short and clear, like a helpful note.

Use assert messages especially in bigger tests with many checks.

Summary

Assertions check if something is true in tests.

Adding messages explains why a test failed.

Use clear messages to save time debugging.