Regression testing in Testing Fundamentals - Build an Automation Script

Verify that the login functionality works after recent code changes

Preconditions (2)

Step 1: Open the application login page

Step 2: Enter 'testuser' in the username field

Step 3: Enter 'Test@1234' in the password field

Step 4: Click the 'Login' button

Step 5: Observe the page after login

✅ Expected Result: User is successfully logged in and redirected to the dashboard page with URL containing '/dashboard'

Automation Requirements - Selenium with Python

Assertions Needed:

Verify the current URL contains '/dashboard' after login

Verify that the logout button is visible on the dashboard page

Best Practices:

Use explicit waits to wait for elements to be visible or clickable

Use Page Object Model to separate page locators and actions

Use meaningful and stable locators like IDs or data-test attributes

Avoid hardcoded sleeps

Clean up by closing the browser after test

Automated Solution

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

class LoginPage:
    def __init__(self, driver):
        self.driver = driver
        self.username_input = (By.ID, 'username')
        self.password_input = (By.ID, 'password')
        self.login_button = (By.ID, 'loginBtn')

    def enter_username(self, username):
        WebDriverWait(self.driver, 10).until(EC.visibility_of_element_located(self.username_input)).send_keys(username)

    def enter_password(self, password):
        WebDriverWait(self.driver, 10).until(EC.visibility_of_element_located(self.password_input)).send_keys(password)

    def click_login(self):
        WebDriverWait(self.driver, 10).until(EC.element_to_be_clickable(self.login_button)).click()

class DashboardPage:
    def __init__(self, driver):
        self.driver = driver
        self.logout_button = (By.ID, 'logoutBtn')

    def is_logout_visible(self):
        return WebDriverWait(self.driver, 10).until(EC.visibility_of_element_located(self.logout_button)) is not None


def test_login_functionality():
    driver = webdriver.Chrome()
    driver.get('https://example.com/login')

    login_page = LoginPage(driver)
    login_page.enter_username('testuser')
    login_page.enter_password('Test@1234')
    login_page.click_login()

    # Verify URL contains '/dashboard'
    WebDriverWait(driver, 10).until(EC.url_contains('/dashboard'))
    assert '/dashboard' in driver.current_url, f"Expected '/dashboard' in URL but got {driver.current_url}"

    dashboard_page = DashboardPage(driver)
    assert dashboard_page.is_logout_visible(), "Logout button should be visible on dashboard"

    driver.quit()

if __name__ == '__main__':
    test_login_functionality()

This script uses Selenium with Python to automate the regression test for login functionality.

We use the Page Object Model to keep locators and actions organized in LoginPage and DashboardPage classes.

Explicit waits ensure the script waits for elements to appear or be clickable, avoiding flaky tests.

The test opens the login page, enters credentials, clicks login, then verifies the URL contains '/dashboard' and the logout button is visible.

Finally, the browser is closed to clean up.

Common Mistakes - 4 Pitfalls

Using hardcoded sleep like time.sleep(5) instead of explicit waits

Using brittle locators like absolute XPath

Not closing the browser after test

Mixing test logic and page locators in one place

Bonus Challenge

Now add data-driven testing with 3 different sets of valid username and password combinations

Show Hint

Practice

1.

What is the main purpose of regression testing in data science projects?

easy

A. To check that new changes do not break existing results

B. To create new models from scratch

C. To collect new data for training

D. To visualize data trends

Solution

1. Step 1: Understand regression testing goal

   Regression testing is about verifying that recent changes do not cause errors in existing functionality.

2. Step 2: Match purpose with options

   Only To check that new changes do not break existing results describes this goal correctly, others describe unrelated tasks.

3. Final Answer:

   To check that new changes do not break existing results -> Option A

4. Quick Check:

   Regression testing = check no breakage [OK]

Hint: Regression testing ensures old results still work after changes [OK]

Common Mistakes:

• Confusing regression testing with model building
• Thinking it collects new data
• Assuming it is for data visualization

2.

Which of the following is the correct way to describe regression testing?

Regression testing:

def regression_test(old_output, new_output):
    return old_output == new_output

easy

A. It compares old and new outputs to detect changes

B. It trains a new model using old and new data

C. It visualizes differences between datasets

D. It deletes old test results before running new tests

Solution

1. Step 1: Analyze the function code

   The function returns True if old_output equals new_output, meaning it compares outputs.

2. Step 2: Match function behavior to options

   It compares old and new outputs to detect changes correctly states it compares outputs to detect changes; others describe unrelated actions.

3. Final Answer:

   It compares old and new outputs to detect changes -> Option A

4. Quick Check:

   Function compares outputs = It compares old and new outputs to detect changes [OK]

Hint: Regression test checks if old and new outputs match exactly [OK]

Common Mistakes:

• Thinking regression testing retrains models
• Assuming it creates visualizations
• Believing it deletes old results

3.

Given the following Python code for regression testing outputs, what will be the printed result?

old_results = [10, 20, 30]
new_results = [10, 20, 25]

if old_results == new_results:
    print("Test Passed")
else:
    print("Test Failed")

medium

A. Test Passed

B. SyntaxError

C. Test Failed

D. No output

Solution

1. Step 1: Compare the two lists element-wise

   old_results has [10, 20, 30], new_results has [10, 20, 25]. The last elements differ (30 vs 25).

2. Step 2: Evaluate the if condition

   Since the lists are not equal, the else block runs, printing "Test Failed".

3. Final Answer:

   Test Failed -> Option C

4. Quick Check:

   Lists differ, so output = Test Failed [OK]

Hint: Lists must match exactly for test to pass [OK]

Common Mistakes:

• Assuming partial match passes test
• Confusing list equality with set equality
• Expecting syntax errors from correct code

4.

Identify the error in this regression testing code snippet:

def regression_test(old, new):
    if old = new:
        return True
    else:
        return False

medium

A. Missing return statement

B. Using assignment operator '=' instead of comparison '==' in if condition

C. Function should print results, not return

D. Incorrect function name

Solution

1. Step 1: Check the if condition syntax

   The condition uses '=' which is assignment, not comparison. Python requires '==' for comparison.

2. Step 2: Identify correct syntax for comparison

   Replacing '=' with '==' fixes the syntax error and allows proper comparison.

3. Final Answer:

   Using assignment operator '=' instead of comparison '==' in if condition -> Option B

4. Quick Check:

   Use '==' to compare, not '=' [OK]

Hint: Use '==' for comparison, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Thinking return is missing
• Believing function name matters for syntax

5.

You have a data science model that outputs predictions as a list. You want to automate regression testing to check if new code changes break the model's output. Which approach below best applies regression testing?

hard

A. Delete old predictions and start fresh after every change

B. Train a new model with updated data and ignore old predictions

C. Visualize new predictions only to check for differences

D. Save previous predictions and compare them with new predictions after changes

Solution

1. Step 1: Understand regression testing in model outputs

   Regression testing means re-running tests and comparing new outputs to old outputs to detect unintended changes.

2. Step 2: Evaluate options for best fit

   Save previous predictions and compare them with new predictions after changes saves old predictions and compares them to new ones, matching regression testing. Others ignore old outputs or delete them, which is not regression testing.

3. Final Answer:

   Save previous predictions and compare them with new predictions after changes -> Option D

4. Quick Check:

   Regression testing = compare old and new outputs [OK]

Hint: Keep old outputs to compare with new after changes [OK]

Common Mistakes:

• Ignoring old outputs in testing
• Assuming visualization replaces regression tests
• Deleting old results before testing