Are you preparing for a FastAPI interview and want to boost your chances of landing the job? You’ve come to the right place! In this comprehensive guide, we cover the top 25 FastAPI interview questions that are commonly asked by leading tech companies. Whether you’re a backend developer, Python engineer, or API enthusiast, these questions and detailed answers—with real-world examples—will help you understand FastAPI concepts inside out.

FastAPI is quickly becoming one of the most popular Python web frameworks due to its blazing speed, ease of use, and automatic documentation features. Knowing how FastAPI works under the hood can set you apart in technical interviews. Let’s dive into the essential questions every FastAPI developer should master in 2025.

1. What is FastAPI and what are its key features?

FastAPI is a modern, high-performance web framework for building APIs with Python 3.7+ based on standard Python type hints.

Key features:

• Fast: High performance, comparable to Node.js and Go.
  
  
• Easy to use: Fewer bugs, faster development.
  
  
• Automatic docs: Swagger UI and ReDoc.
  
  
• Type safety: Uses Python type hints for validation.
  
  
• Asynchronous support: Built on Starlette and supports async/await.
  
  

Example:

from fastapi import FastAPI

app = FastAPI()

@app.get(“/”)

def read_root():

    return {“Hello”: “World”}

2. How does FastAPI perform automatic validation?

FastAPI uses Pydantic for data validation and serialization based on Python type hints.

Example:

from pydantic import BaseModel

class Item(BaseModel):

    name: str

    price: float

    is_offer: bool = None

@app.post(“/items/”)

def create_item(item: Item):

    return item

This will automatically validate the request body.

3. Explain the role of Pydantic in FastAPI.

 Pydantic is used for:

• Parsing and validating request/response data.
  
  
• Generating JSON Schema for documentation.
  
  
• Enforcing data types using Python type hints.
  
  

4. How does FastAPI support asynchronous programming?

FastAPI natively supports async functions using async def, leveraging Python’s asyncio.

Example:

@app.get(“/async”)

async def read_async():

    await some_async_operation()

    return {“message”: “Async operation complete”}

5. How are path parameters defined in FastAPI?

@app.get(“/items/{item_id}”)

def read_item(item_id: int):

    return {“item_id”: item_id}

FastAPI automatically converts and validates item_id as an integer.

6. How can you handle query parameters in FastAPI?

@app.get(“/items/”)

def read_item(skip: int = 0, limit: int = 10):

    return {“skip”: skip, “limit”: limit}

These are optional query parameters with default values.

7. How do you document APIs in FastAPI?

FastAPI auto-generates documentation using:

• Swagger UI: at /docs
  
  
• ReDoc: at /redoc
  
  

8. What is dependency injection in FastAPI?

Dependency injection allows you to share logic across routes, like authentication or DB sessions.

Example:

from fastapi import Depends

def common_dep():

    return “common dependency”

@app.get(“/items/”)

def read_items(dep=Depends(common_dep)):

    return {“dep”: dep}

9. How do you handle request bodies?

Use Pydantic models to declare the request body schema.

@app.post(“/item/”)

def create_item(item: Item):

    return item

10. How do you use response models in FastAPI?

@app.post(“/item/”, response_model=Item)

def create_item(item: Item):

    return item

This restricts the response to the Item schema.

11. How do you return custom status codes?

from fastapi import status

@app.post(“/item/”, status_code=status.HTTP_201_CREATED)

def create_item(item: Item):

    return item

12. How do you use middleware in FastAPI?

from fastapi import Request

from starlette.middleware.base import BaseHTTPMiddleware

class SimpleMiddleware(BaseHTTPMiddleware):

    async def dispatch(self, request: Request, call_next):

        response = await call_next(request)

        return response

app.add_middleware(SimpleMiddleware)

13. How do you handle exceptions globally?

from fastapi import Request, HTTPException

from fastapi.responses import JSONResponse

from fastapi.exception_handlers import RequestValidationError

@app.exception_handler(HTTPException)

async def http_exception_handler(request: Request, exc: HTTPException):

    return JSONResponse(status_code=exc.status_code, content={“message”: str(exc.detail)})

14. How can you upload files using FastAPI?

from fastapi import File, UploadFile

@app.post(“/upload/”)

async def upload_file(file: UploadFile = File(…)):

    content = await file.read()

    return {“filename”: file.filename}

15. How do you use Background Tasks in FastAPI?

from fastapi import BackgroundTasks

def write_log(message: str):

    with open(“log.txt”, “a”) as f:

        f.write(message)

@app.post(“/log/”)

def log_message(message: str, background_tasks: BackgroundTasks):

    background_tasks.add_task(write_log, message)

    return {“message”: “Log scheduled”}

16. How do you implement authentication in FastAPI?

Use OAuth2PasswordBearer for token-based authentication.

Example:

from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl=”token”)

@app.get(“/secure/”)

def secure_data(token: str = Depends(oauth2_scheme)):

    return {“token”: token}

17. How do you serve static files?

from fastapi.staticfiles import StaticFiles

app.mount(“/static”, StaticFiles(directory=”static”), name=”static”)

18. How do you create custom response types?

from fastapi.responses import HTMLResponse

@app.get(“/html”, response_class=HTMLResponse)

def get_html():

    return “<h1>Hello, FastAPI</h1>”

19. What are the different ways to send responses?

• JSON (default)
  
  
• PlainTextResponse
  
  
• HTMLResponse
  
  
• FileResponse
  
  
• StreamingResponse
  
  
• RedirectResponse
  
  

20. What is Starlette and how does it relate to FastAPI?

FastAPI is built on top of Starlette, which provides the ASGI web toolkit including routing, middleware, and requests/responses.

21. How do you handle CORS in FastAPI?

from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(

    CORSMiddleware,

    allow_origins=[“*”],

    allow_methods=[“*”],

    allow_headers=[“*”],

)

22. How do you test FastAPI applications?

Use TestClient from fastapi.testclient.

Example:

from fastapi.testclient import TestClient

client = TestClient(app)

def test_read_root():

    response = client.get(“/”)

    assert response.status_code == 200

    assert response.json() == {“Hello”: “World”}

23. How do you use environment variables with FastAPI?

Use python-dotenv or os.environ.

import os

from dotenv import load_dotenv

load_dotenv()

API_KEY = os.getenv(“API_KEY”)

24. How do you structure a large FastAPI project?

• main.py: entry point
  
  
• routers/: API routers
  
  
• models/: Pydantic/DB models
  
  
• schemas/: request/response models
  
  
• services/: business logic
  
  
• core/: configuration, security
  
  

25. How do you connect to a database in FastAPI?

With SQLAlchemy:

from sqlalchemy.orm import sessionmaker

SessionLocal = sessionmaker(bind=engine)

def get_db():

    db = SessionLocal()

    try:

        yield db

    finally:

        db.close()

Use Depends(get_db) to inject the DB session.

Mastering FastAPI is a smart career move for any Python developer in today’s job market. This list of the top 25 FastAPI interview questions and answers is designed to help you confidently tackle any technical screening, from startups to large tech firms. We covered key topics like routing, validation, dependency injection, authentication, async support, and more—with code examples to solidify your understanding.

Make sure to practice these concepts and build small projects to reinforce your learning. The more hands-on experience you have, the better you’ll perform in interviews. For more developer tips, backend interview guides, and FastAPI tutorials, stay tuned to our blog.