FastAPI Interview Questions (Beginner to Advanced)
🧠 1. Basics of FastAPI
- What is FastAPI?
- Why is FastAPI faster compared to Flask or Django?
- What are the key features of FastAPI?
- Is FastAPI synchronous or asynchronous?
- What is ASGI and how is it related to FastAPI?
⚙️ 2. Core Concepts
- What is a path operation in FastAPI?
- Difference between GET, POST, PUT, DELETE methods?
- What are path parameters and query parameters?
- What is request body in FastAPI?
- What is Pydantic and why is it used?
🧾 3. Request & Response Handling
- How does FastAPI validate request data?
- What is a response model?
- How do you return JSON in FastAPI?
- What are status codes in FastAPI?
- How do you handle form data?
🔐 4. Authentication & Security
- How do you implement authentication in FastAPI?
- What is OAuth2 in FastAPI?
- What is JWT and how is it used?
- Difference between OAuth2PasswordBearer and API Key authentication?
- How do you secure APIs in FastAPI?
🧩 5. Dependency Injection
- What is dependency injection in FastAPI?
- Why is Depends() used?
- How does FastAPI manage dependencies?
- Example of using dependencies in routes?
🗄️ 6. Database Integration
- How do you connect FastAPI with a database?
- What is SQLAlchemy?
- Difference between ORM and raw SQL?
- How do you create models in FastAPI?
- What is Alembic used for?
⚡ 7. Async & Performance
- What is async and await in FastAPI?
- When should you use async functions?
- How does FastAPI handle concurrency?
- What is event loop?
- Is FastAPI suitable for high-performance applications?
🧪 8. Testing & Debugging
- How do you test FastAPI applications?
- What is TestClient?
- How do you handle exceptions in FastAPI?
- What is middleware in FastAPI?
📦 9. Advanced Topics
- What is middleware in FastAPI?
- How do you handle CORS in FastAPI?
- What is background task in FastAPI?
- What is WebSocket in FastAPI?
- How do you deploy FastAPI applications?
💡 10. Coding Questions (Very Common)
- Create a simple CRUD API using FastAPI
- Build an API for student management
- Validate user input using Pydantic
- Create login API using JWT
- Connect FastAPI with SQLite/PostgreSQL
FastAPI Interview Answers
🧠 1. Basics
Q: What is FastAPI?
FastAPI is a modern Python web framework used to build APIs quickly and efficiently using Python type hints.
Q: Why is FastAPI faster than Flask/Django?
Because it is built on ASGI (Asynchronous Server Gateway Interface) and supports async/await, enabling high concurrency.
Q: Is FastAPI synchronous or asynchronous?
It supports both, but it is designed mainly for asynchronous programming.
Q: What is ASGI?
ASGI is a specification that allows asynchronous communication between web servers and Python applications.
⚙️ 2. Core Concepts
Q: What is a path operation?
It is a combination of HTTP method (GET, POST, etc.) and a URL route in FastAPI.
Q: Path parameters vs Query parameters?
- Path:
/users/1→ required in URL - Query:
/users?id=1→ optional parameters
Q: What is request body?
Data sent from client to server in POST/PUT requests, usually in JSON format.
Q: What is Pydantic?
A library used for data validation and parsing using Python type hints.
🧾 3. Request & Response
Q: What is response model?
A schema that defines the structure of API output using Pydantic models.
Q: How does FastAPI validate data?
It uses Pydantic models automatically to validate incoming requests.
Q: How do you return JSON?
FastAPI automatically converts Python dictionaries to JSON responses.
🔐 4. Authentication
Q: How do you implement authentication?
Using OAuth2, JWT tokens, or API keys with FastAPI security utilities.
Q: What is JWT?
JSON Web Token used for secure user authentication and session management.
🧩 5. Dependency Injection
Q: What is Depends()?
It is used to inject reusable logic (like database connection or auth checks) into routes.
🗄️ 6. Database
Q: How do you connect FastAPI to a database?
Using ORMs like SQLAlchemy or async drivers like Databases library.
Q: ORM vs SQL?
- ORM → object-based database access
- SQL → raw query writing
⚡ 7. Async
Q: What is async/await?
It allows non-blocking execution of code, improving performance.
Q: When to use async?
Use it for I/O tasks like database queries, API calls, file handling.
🧪 8. Testing
Q: How to test FastAPI?
Using TestClient from FastAPI/Starlette.
📦 9. Advanced
Q: What is middleware?
Code that runs before or after every request (e.g., logging, authentication).
Q: What is CORS?
Cross-Origin Resource Sharing — controls which domains can access your API.
💻 10. Coding Example (Very Common)
Q: Create a simple API
from fastapi import FastAPIapp = FastAPI()@app.get("/")
def home():
return {"message": "Hello World"}