Why integration tests verify components together in PyTest - Test Execution Impact

Test Overview

This test checks that two components, a calculator and a logger, work correctly when used together. It verifies that the calculator adds numbers and the logger records the operation.

Test Code - pytest

PyTest

import pytest

class Calculator:
    def __init__(self, logger):
        self.logger = logger

    def add(self, a, b):
        result = a + b
        self.logger.log(f"Added {a} and {b} to get {result}")
        return result

class Logger:
    def __init__(self):
        self.messages = []

    def log(self, message):
        self.messages.append(message)

@pytest.fixture
def logger():
    return Logger()

@pytest.fixture
def calculator(logger):
    return Calculator(logger)


def test_calculator_add_logs_operation(calculator, logger):
    result = calculator.add(3, 4)
    assert result == 7
    assert logger.messages[-1] == "Added 3 and 4 to get 7"

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	Fixtures create Logger and Calculator instances	Logger with empty messages list, Calculator linked to Logger	-	PASS
3	Calculator.add(3, 4) is called	Calculator computes sum 7	-	PASS
4	Logger.log records message 'Added 3 and 4 to get 7'	Logger messages list contains one entry	Logger messages list last entry is 'Added 3 and 4 to get 7'	PASS
5	Assert result == 7	Result is 7	7 == 7	PASS
6	Assert logger message is correct	Logger messages list last entry is 'Added 3 and 4 to get 7'	Message matches expected string	PASS
7	Test ends	All assertions passed	-	PASS

Failure Scenario

Failing Condition: Logger does not record the addition message or Calculator returns wrong sum

Execution Trace Quiz - 3 Questions

What does the integration test verify?

A That Calculator and Logger work correctly together

B That Calculator adds numbers correctly alone

C That Logger stores messages correctly alone

D That Calculator subtracts numbers correctly

Key Result

Integration tests check that separate parts work together correctly, catching problems that unit tests alone might miss.