Why testing validates contracts in Rest API - See It in Action

Why Testing Validates Contracts in REST APIs

📖 Scenario: You are working on a simple REST API that provides user information. The API contract specifies that when you request a user by ID, you get a JSON response with id, name, and email. Testing helps ensure the API follows this contract exactly.

🎯 Goal: Build a simple test that checks if the API response matches the expected contract. This shows how testing validates that the API behaves as promised.

📋 What You'll Learn

Create a sample API response dictionary with user data

Define the expected contract keys in a list

Write a test that checks if all contract keys exist in the response

Print the test result as 'Contract Validated' or 'Contract Failed'

💡 Why This Matters

🌍 Real World

APIs must follow contracts so different programs can work together without errors. Testing these contracts prevents bugs and confusion.

💼 Career

Many software jobs require writing tests to verify APIs meet their contracts, ensuring reliable communication between services.

1

Create the sample API response data

Create a dictionary called api_response with these exact entries: 'id': 101, 'name': 'Alice', 'email': '[e-mail]', and 'age': 30.

Rest API

# Your code here

Hint

Use curly braces {} to create a dictionary with the exact keys and values.

2

Define the expected contract keys

Create a list called contract_keys with these exact strings: 'id', 'name', and 'email'.

Rest API

api_response = {'id': 101, 'name': 'Alice', 'email': '[e-mail]', 'age': 30}
# Your code here

Hint

Use square brackets [] to create a list with the exact strings.

3

Write the test to check the contract

Write a for loop using key to iterate over contract_keys. Inside the loop, check if key is not in api_response. If missing, set a variable valid to False and break the loop. Before the loop, set valid to True.

Rest API

api_response = {'id': 101, 'name': 'Alice', 'email': '[e-mail]', 'age': 30}
contract_keys = ['id', 'name', 'email']
# Your code here

Hint

Start with valid = True. Use a for loop and if to check keys. Use break to stop if a key is missing.

4

Print the test result

Write a print statement that prints 'Contract Validated' if valid is True, otherwise prints 'Contract Failed'.

Rest API

api_response = {'id': 101, 'name': 'Alice', 'email': '[e-mail]', 'age': 30}
contract_keys = ['id', 'name', 'email']
valid = True
for key in contract_keys:
    if key not in api_response:
        valid = False
        break
# Your code here

Hint

Use a single print with a conditional expression to show the result.

Practice

1. Why is testing important for validating REST API contracts?

easy

A. It makes the API run faster by optimizing code.

B. It automatically generates API documentation.

C. It ensures the API follows agreed rules and behaves as expected.

D. It replaces the need for API design.

Solution

1. Step 1: Understand the purpose of API contracts

   API contracts define rules about how APIs should behave and what data they exchange.

2. Step 2: Role of testing in contracts

   Testing checks if the API follows these rules and returns expected results, catching mistakes early.

3. Final Answer:

   It ensures the API follows agreed rules and behaves as expected. -> Option C

4. Quick Check:

   Testing validates contracts = Ensures API rules followed [OK]

Hint: Testing checks if API follows rules and returns expected data [OK]

Common Mistakes:

• Thinking testing speeds up API automatically
• Confusing testing with documentation generation
• Believing testing replaces API design

2. Which of the following is the correct way to test a REST API contract for a GET request?

easy

A. Send a DELETE request and expect a 500 error.

B. Send a POST request and check if the response status is 404.

C. Send a GET request and ignore the response content.

D. Send a GET request and check if the response status is 200 and data format matches the contract.

Solution

1. Step 1: Identify correct HTTP method and expected response

   For a GET request, the API should return status 200 and data matching the contract.

2. Step 2: Match testing action to contract validation

   Sending GET and checking status 200 plus data format confirms contract adherence.

3. Final Answer:

   Send a GET request and check if the response status is 200 and data format matches the contract. -> Option D

4. Quick Check:

   GET test checks status 200 and data format [OK]

Hint: Test GET by checking status 200 and correct data format [OK]

Common Mistakes:

• Using wrong HTTP method for the test
• Ignoring response content in tests
• Expecting error codes for successful GET

3. Given this test code snippet for a REST API endpoint:

response = client.get('/users/123')
assert response.status_code == 200
assert 'id' in response.json()
assert response.json()['id'] == 123

What does this test validate about the API contract?

medium

A. The API returns status 200 and correct user id in JSON response.

B. The API returns status 404 for missing users.

C. The API accepts POST requests at /users/123.

D. The API returns an empty JSON response.

Solution

1. Step 1: Analyze the test assertions

   The test checks that the response status code is 200 and the JSON response contains an 'id' key equal to 123.

2. Step 2: Understand what this means for the API contract

   This confirms the API returns successful status and correct user data as per contract.

3. Final Answer:

   The API returns status 200 and correct user id in JSON response. -> Option A

4. Quick Check:

   Test checks status 200 and user id = 123 [OK]

Hint: Check status and key-value in JSON to validate contract [OK]

Common Mistakes:

• Confusing status 404 with 200
• Assuming POST method is tested here
• Ignoring JSON content checks

4. A test for a REST API endpoint expects a JSON response with a 'name' field. The test code is:

response = client.get('/items/5')
assert response.status_code == 200
assert 'name' in response.json()

But the test fails with a KeyError. What is the likely cause?

medium

A. The response status code is not 200.

B. The API response JSON does not include the 'name' field as per contract.

C. The test used the wrong HTTP method.

D. The client.get() method is not defined.

Solution

1. Step 1: Understand the test failure

   A KeyError means the 'name' key is missing in the JSON response.

2. Step 2: Identify contract violation

   The API response does not include the expected 'name' field, violating the contract.

3. Final Answer:

   The API response JSON does not include the 'name' field as per contract. -> Option B

4. Quick Check:

   Missing 'name' key causes KeyError [OK]

Hint: KeyError means expected JSON key missing in response [OK]

Common Mistakes:

• Assuming wrong HTTP method caused error
• Ignoring status code correctness
• Blaming client.get() method existence

5. You have a REST API contract that requires the endpoint /orders/{id} to return a JSON with fields id, status, and items (a list). Which test best validates this contract?

hard

A. Send GET /orders/10, check status 200, verify JSON has 'id', 'status', and 'items' keys, and 'items' is a list.

B. Send POST /orders with empty body and expect status 201.

C. Send GET /orders/10 and check only the status code is 404.

D. Send DELETE /orders/10 and expect status 500.

Solution

1. Step 1: Identify contract requirements

   The contract requires GET /orders/{id} to return JSON with 'id', 'status', and 'items' keys, where 'items' is a list.

2. Step 2: Match test to contract validation

   Sending GET /orders/10, checking status 200, verifying JSON has 'id', 'status', and 'items' keys, and confirming 'items' is a list fully validates the contract.

3. Final Answer:

   Send GET /orders/10, check status 200, verify JSON has 'id', 'status', and 'items' keys, and 'items' is a list. -> Option A

4. Quick Check:

   Test checks all required fields and types [OK]

Hint: Check all required fields and data types in JSON response [OK]

Common Mistakes:

• Testing wrong HTTP method
• Ignoring required JSON fields
• Not verifying data types like list