Prev Next

Python / FastAPI Basics Interview Questions

How do you receive and validate a JSON request body in FastAPI?

Declare a Pydantic BaseModel as a function parameter. FastAPI automatically reads the JSON body, validates it against the model, and provides it as a typed Python object. If validation fails it returns a 422 with details.

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Annotated
from pydantic import Field

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = None  # optional field
    price: float
    tax: float | None = None

@app.post("/items")
def create_item(item: Item):  # body parsed + validated automatically
    total = item.price + (item.tax or 0)
    return {**item.model_dump(), "total_price": total}

# Mixing path param + query param + body in the same endpoint
@app.put("/items/{item_id}")
def update_item(
    item_id: int,          # path
    q: str | None = None,  # query
    item: Item | None = None,  # body (optional)
):
    result = {"item_id": item_id}
    if q:
        result["q"] = q
    if item:
        result.update(item.model_dump())
    return result

FastAPI resolves the source of each parameter automatically:

Parameter source resolution
Parameter typeSource
Matches a path segment {name}Path parameter
Simple type (int, str, float…) + not in pathQuery parameter
Pydantic BaseModel subclassJSON request body
Annotated with Body()Explicitly JSON body
What HTTP status code does FastAPI return when request body validation fails?
If a FastAPI endpoint has a parameter typed as a Pydantic BaseModel subclass, where does FastAPI read it from?

More Related questions...

What is FastAPI and what are its key advantages over Flask or Django REST Framework? How do you create and run a minimal FastAPI application? What is the difference between path parameters and query parameters in FastAPI? How do you receive and validate a JSON request body in FastAPI? How do you use Pydantic models for data validation and what validation features does FastAPI support? What is the response_model parameter in FastAPI and why should you use it? How do you add validation constraints to path and query parameters using Path() and Query()? How do you control HTTP status codes and return custom responses in FastAPI? What is FastAPI's dependency injection system and how do you use it? How do you organise a FastAPI application with multiple routers (APIRouter)? What is middleware in FastAPI and how do you add custom middleware? When should you use async def vs def for route handlers in FastAPI? What are BackgroundTasks in FastAPI and when should you use them? How do you implement OAuth2 password flow with JWT tokens in FastAPI? How do you implement role-based access control (RBAC) using FastAPI dependencies? How do you integrate an async SQLAlchemy database with FastAPI? How do you manage database schema migrations in a FastAPI project with Alembic? How do you write tests for a FastAPI application using pytest and TestClient? How do you create custom exception handlers in FastAPI? How do you handle form data and file uploads in FastAPI? How do you manage environment variables and settings in FastAPI with Pydantic Settings? How do you run startup and shutdown logic in FastAPI using lifespan? How do you implement WebSocket endpoints in FastAPI? How do you containerise and deploy a FastAPI application with Docker? What are the key production deployment considerations for a FastAPI application? What are the key differences between Pydantic v1 and v2, and how does FastAPI use Pydantic v2? How do you add caching to FastAPI endpoints to improve performance? How do you customise the OpenAPI documentation in FastAPI? How do you integrate FastAPI with Celery for reliable background task processing? How do you measure and improve the performance of a FastAPI application? How do you use class-based dependencies and sub-dependencies in FastAPI? How do you test async FastAPI endpoints and async dependencies? How do you stream large responses in FastAPI using StreamingResponse? How do you add GraphQL support to a FastAPI application with Strawberry? How does FastAPI handle validation errors and how can you customise the error response format? What is the scope of a FastAPI dependency, and how do you share state across requests? How do you read HTTP headers and cookies in FastAPI? What are the most important FastAPI best practices for a production-ready API?
Show more question and Answers...

PyTorch Fundamentals Interview Questions

Comments & Discussions

Interviews Questions

About Javapedia.net

Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples.

This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company.

Follow Javapedia updates

contact: javatutorials2016[at]gmail[dot]com

Kindly consider donating for maintaining this website. Thanks.

Copyright © 2026, javapedia.net, all rights reserved. · .

DMCA.com Protection Status