FastAPI Interview Questions and Answers (Beginner to Advanced)

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.

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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

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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.

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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.

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4️⃣ What are Pydantic models?

Pydantic models are used for:

• Request body validation

• Response validation

• Type enforcement

Example:

class User(BaseModel):
    name: str
    age: int

FastAPI automatically validates incoming JSON against this model.

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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)

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6️⃣ What is dependency injection in FastAPI?

Dependency Injection (DI) allows reusable components like:

• Database sessions

• Authentication logic

• Common validations

Example:

Depends(get_db)

It improves code reusability and testability.

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7️⃣ How do you handle authentication in FastAPI?

FastAPI supports:

• OAuth2

• JWT

• API Keys

• Basic Auth

It provides built-in security utilities like:

OAuth2PasswordBearer

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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

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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.

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🔟 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()

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1️⃣1️⃣ How do you write unit tests in FastAPI?

FastAPI uses:

• pytest

• TestClient

Example:

def test_health():
    response = client.get("/health")
    assert response.status_code == 200

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1️⃣2️⃣ How is FastAPI deployed in production?

Typical production setup:

Nginx
 ↓
Gunicorn + Uvicorn workers
 ↓
FastAPI app

Often containerized using Docker and orchestrated with Kubernetes.

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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.

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1️⃣4️⃣ What are background tasks in FastAPI?

Background tasks allow execution after response is sent.

Used for:

• Sending emails

• Logging

• Notifications

Example:

BackgroundTasks

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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

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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?

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Final Thoughts

FastAPI is not just a framework, it’s a modern API philosophy:

• Clean code

• High performance

• Strong typing

• Developer-friendly