Why testing prevents costly failures in Testing Fundamentals - Test Execution Impact

Test Overview

This test simulates checking a simple login form to ensure the system catches errors early. It verifies that the login button is clickable and that an error message appears when invalid credentials are entered. This helps prevent costly failures by catching issues before release.

Test Code - Selenium with unittest

Testing Fundamentals

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
import unittest

class TestLoginForm(unittest.TestCase):
    def setUp(self):
        self.driver = webdriver.Chrome()
        self.driver.get('https://example.com/login')

    def test_invalid_login_shows_error(self):
        driver = self.driver
        # Wait for username field
        WebDriverWait(driver, 10).until(EC.presence_of_element_located((By.ID, 'username')))
        username_input = driver.find_element(By.ID, 'username')
        password_input = driver.find_element(By.ID, 'password')
        login_button = driver.find_element(By.ID, 'login-btn')

        username_input.send_keys('wronguser')
        password_input.send_keys('wrongpass')
        login_button.click()

        # Wait for error message
        error = WebDriverWait(driver, 10).until(
            EC.visibility_of_element_located((By.ID, 'error-msg'))
        )
        self.assertTrue(error.is_displayed())
        self.assertEqual(error.text, 'Invalid username or password.')

    def tearDown(self):
        self.driver.quit()

if __name__ == '__main__':
    unittest.main()

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts and opens Chrome browser	Browser window opens at 'https://example.com/login' showing login form with username, password fields and login button	-	PASS
2	Waits until username input field is present	Username input field is visible and ready for input	Presence of element with ID 'username'	PASS
3	Finds username, password inputs and login button elements	All three elements found on the page	-	PASS
4	Enters invalid username and password	Username field contains 'wronguser', password field contains 'wrongpass'	-	PASS
5	Clicks the login button	Login form submits and page processes login attempt	-	PASS
6	Waits for error message to appear	Error message element with ID 'error-msg' becomes visible	Visibility of element with ID 'error-msg'	PASS
7	Checks that error message is displayed and text matches expected	Error message shown: 'Invalid username or password.'	error.is_displayed() is True and error.text equals 'Invalid username or password.'	PASS
8	Test ends and browser closes	Browser window closes	-	PASS

Failure Scenario

Failing Condition: Error message does not appear after submitting invalid credentials

Execution Trace Quiz - 3 Questions

What does the test verify after clicking the login button?

A That the user is redirected to the homepage

B That an error message appears for invalid login

C That the login button becomes disabled

D That the password field is cleared

Key Result

Testing early by verifying error messages on invalid input helps catch issues before release, preventing costly failures in production.