FastAPI has quickly become one of the most popular Python frameworks for building high-performance APIs. As a result, FastAPI interview questions are now common in backend, microservices, and AI/ML roles.
This post covers commonly asked FastAPI interview questions with simple and clear answers, from basic concepts to advanced topics.
1️⃣ What is FastAPI?
FastAPI is a modern, high-performance Python web framework used to build REST APIs quickly.
It is built on Starlette for web handling and Pydantic for data validation.
Key features:
-
Fast performance
-
Automatic API documentation
-
Built-in request validation
-
Async support
2️⃣ Why is FastAPI faster than Flask or Django?
FastAPI is faster because:
-
It uses ASGI instead of WSGI
-
Supports async/await
-
Uses Uvicorn (high-performance server)
-
Minimizes blocking IO
This makes FastAPI ideal for high-concurrency APIs.
3️⃣ What is ASGI and how is it different from WSGI?
| WSGI | ASGI |
|---|---|
| Synchronous | Asynchronous |
| One request per thread | Event-loop based |
| Used by Flask, Django | Used by FastAPI |
FastAPI uses ASGI, enabling better scalability.
4️⃣ What are Pydantic models?
Pydantic models are used for:
-
Request body validation
-
Response validation
-
Type enforcement
Example:
FastAPI automatically validates incoming JSON against this model.
5️⃣ How does FastAPI generate Swagger documentation?
FastAPI automatically generates OpenAPI (Swagger) documentation using:
-
Python type hints
-
Route definitions
-
Pydantic models
Available at:
-
/docs(Swagger UI) -
/redoc(ReDoc)
6️⃣ What is dependency injection in FastAPI?
Dependency Injection (DI) allows reusable components like:
-
Database sessions
-
Authentication logic
-
Common validations
Example:
It improves code reusability and testability.
7️⃣ How do you handle authentication in FastAPI?
FastAPI supports:
-
OAuth2
-
JWT
-
API Keys
-
Basic Auth
It provides built-in security utilities like:
8️⃣ What is the difference between def and async def in FastAPI?
| def | async def |
|---|---|
| Blocking | Non-blocking |
| CPU-bound tasks | IO-bound tasks |
| Simpler | High concurrency |
Use async def when calling:
-
Databases
-
External APIs
-
Message queues
9️⃣ How does FastAPI handle request validation errors?
FastAPI:
-
Automatically validates requests
-
Returns HTTP 422 (Unprocessable Entity)
-
Provides clear error messages
No manual validation code is required.
🔟 How do you connect FastAPI to a database?
FastAPI supports:
-
SQLAlchemy (SQL)
-
Motor (MongoDB)
-
Async ORMs
Typically:
-
Create a DB session dependency
-
Inject it using
Depends()
1️⃣1️⃣ How do you write unit tests in FastAPI?
FastAPI uses:
-
pytest -
TestClient
Example:
1️⃣2️⃣ How is FastAPI deployed in production?
Typical production setup:
Often containerized using Docker and orchestrated with Kubernetes.
1️⃣3️⃣ How does FastAPI compare to Spring Boot?
| FastAPI | Spring Boot |
|---|---|
| Python | Java |
| Async-first | Thread-based |
| Minimal config | Heavy config |
| Fast development | Enterprise stability |
Many companies use both together.
1️⃣4️⃣ What are background tasks in FastAPI?
Background tasks allow execution after response is sent.
Used for:
-
Sending emails
-
Logging
-
Notifications
Example:
1️⃣5️⃣ What are common FastAPI interview mistakes?
❌ Using def instead of async def
❌ Blocking calls inside async routes
❌ Ignoring dependency injection
❌ Not handling exceptions properly
❌ Poor project structure
Advanced FastAPI Interview Questions (For Experienced Devs)
-
How does FastAPI handle concurrency internally?
-
How do you implement rate limiting?
-
How do you use middleware in FastAPI?
-
How do you secure APIs using OAuth2 + JWT?
-
How do you scale FastAPI horizontally?
-
How do you integrate FastAPI with Kafka / RabbitMQ?
Final Thoughts
FastAPI is not just a framework, it’s a modern API philosophy:
-
Clean code
-
High performance
-
Strong typing
-
Developer-friendly