API Testing Fundamentals

What is API testing?

API testing validates a service at the HTTP interface level:

• requests and responses
• status codes and headers
• payload correctness (JSON shape)
• authentication/authorization
• error handling and rate limits

What to test (checklist)

• Status codes: 200/201/204, 400/401/403/404, 429, 5xx
• Response body: required keys, value types, constraints
• Contracts: backwards compatibility
• Auth: missing token, invalid token, role permissions
• Idempotency: repeated requests behave correctly
• Performance: basic response time budgets

Diagram: test levels around an API

diagram Diagram mermaid

  graph TD
A["Unit tests\n(request builders, validators)"] --> B["API tests\n(HTTP-level)"]
B --> C["End-to-end tests\n(user workflows)"]

Tip

• Write API tests as deterministic as possible.
• Avoid relying on external services in CI.

Visualize it

An API test builds a request, sends it to the endpoint, and checks the response before reporting pass or fail.

diagram API Test Flow mermaid

  flowchart LR
A["Build request"] --> B["Send to endpoint"]
B --> C["Receive response"]
C --> D["Assert status code + body"]
D --> E["Pass"]
D --> F["Fail"]

An API test builds a request, sends it, checks the response, and reports pass or fail.

🧪 Try It Yourself

Exercise 1 – Write a unittest TestCase

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Exercise 2 – assertRaises

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Exercise 3 – setUp and tearDown

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