Shift-left testing in Testing Fundamentals - Build an Automation Script

Automate unit test execution early in development

Preconditions (2)

Step 1: Write a unit test for the function that checks correct output for input 5

Step 2: Run the unit test immediately after writing the function

Step 3: Verify the test passes without errors

✅ Expected Result: The unit test runs successfully and confirms the function returns the expected output for input 5

Automation Requirements - pytest

Assertions Needed:

Verify function output equals expected value

Best Practices:

Write tests close to code (shift-left principle)

Use clear and simple assertions

Keep tests small and focused

Automated Solution

Testing Fundamentals

def multiply_by_two(x):
    return x * 2


def test_multiply_by_two():
    result = multiply_by_two(5)
    assert result == 10, f"Expected 10 but got {result}"

if __name__ == "__main__":
    import pytest
    pytest.main([__file__])

This script defines a simple function multiply_by_two that doubles the input.

The test function test_multiply_by_two calls this function with input 5 and asserts the output is 10.

Running the test immediately after writing the function follows the shift-left testing idea: testing early in development to catch issues quickly.

The assertion includes a helpful message if it fails.

The pytest.main call allows running the test by executing the script directly.

Common Mistakes - 3 Pitfalls

Writing tests only after full feature completion

Using print statements instead of assertions

Testing too many things in one test

Bonus Challenge

Now add data-driven testing with 3 different inputs (2, 5, 10) and expected outputs

Show Hint

Practice

1. What is the main idea behind shift-left testing?

easy

A. Testing only the user interface at the end

B. Testing early in the software development process

C. Testing only after the software is fully developed

D. Testing only by the quality assurance team

Solution

1. Step 1: Understand the term 'shift-left'

   Shift-left means moving testing activities to earlier stages in development.

2. Step 2: Compare options with the definition

   Only Testing early in the software development process describes testing early, others describe late or limited testing.

3. Final Answer:

   Testing early in the software development process -> Option B

4. Quick Check:

   Shift-left = Early testing [OK]

Hint: Shift-left means test early, not late [OK]

Common Mistakes:

• Thinking testing happens only after coding
• Confusing shift-left with testing only UI
• Believing only QA team tests

2. Which of the following is a common practice that supports shift-left testing?

easy

A. Writing tests after the product release

B. Performing only manual testing at the end

C. Test-driven development (TDD)

D. Skipping code reviews

Solution

1. Step 1: Identify practices that enable early testing

   TDD means writing tests before or during coding, supporting early testing.

2. Step 2: Eliminate options that delay or skip testing

   Options A, B, and C delay or reduce testing, not shift it left.

3. Final Answer:

   Test-driven development (TDD) -> Option C

4. Quick Check:

   TDD = Early testing practice [OK]

Hint: TDD means write tests first, fits shift-left [OK]

Common Mistakes:

• Thinking tests come only after release
• Ignoring code reviews as testing support
• Believing manual testing only is enough

3. Consider this code snippet representing a testing timeline:

Development Start -> Code Review -> Unit Testing -> Integration Testing -> Release

Which step best represents the shift-left testing approach?

medium

A. Unit Testing

B. Release

C. Code Review

D. Integration Testing

Solution

1. Step 1: Understand shift-left means testing early

   Shift-left moves testing activities closer to development start.

2. Step 2: Identify earliest testing-related step

   Unit Testing is the earliest formal testing phase after code review, which is a review step but not a testing activity.

3. Final Answer:

   Unit Testing -> Option A

4. Quick Check:

   Shift-left = Early testing phases like unit testing [OK]

Hint: Shift-left means test as early as possible [OK]

Common Mistakes:

• Choosing testing steps that happen later
• Confusing release with testing
• Ignoring code review as testing support

4. A team claims they follow shift-left testing but only start testing after full coding is done. What is the main problem in their approach?

medium

A. They are not testing early enough

B. They are testing too early

C. They are using too many automated tests

D. They are skipping integration testing

Solution

1. Step 1: Recall shift-left means testing early

   Testing after full coding is late, not early.

2. Step 2: Identify the problem with their timing

   The team delays testing, so they miss shift-left benefits.

3. Final Answer:

   They are not testing early enough -> Option A

4. Quick Check:

   Shift-left = test early, not late [OK]

Hint: Shift-left means test early, not after coding done [OK]

Common Mistakes:

• Thinking testing too early is a problem
• Confusing automation with timing
• Ignoring integration testing relevance

5. A project uses shift-left testing by integrating automated unit tests early. However, bugs still appear late in integration testing. What is a likely cause?

hard

A. Manual testing should replace automated tests

B. Automated tests are run too early in development

C. Shift-left testing means skipping integration tests

D. Unit tests do not cover integration points between modules

Solution

1. Step 1: Understand shift-left testing scope

   Shift-left encourages early testing but does not replace integration testing.

2. Step 2: Analyze why bugs appear late despite early unit tests

   Unit tests check individual parts but may miss how parts work together.

3. Step 3: Identify the cause of late bugs

   Missing coverage of integration points causes bugs to appear late.

4. Final Answer:

   Unit tests do not cover integration points between modules -> Option D

5. Quick Check:

   Early unit tests ≠ full integration coverage [OK]

Hint: Unit tests check parts, integration tests check connections [OK]

Common Mistakes:

• Thinking shift-left skips integration testing
• Believing running tests early causes bugs
• Replacing automated tests with manual only