Module 4 — Response Models & Output Validation
FastAPI validates not only incoming data but also outgoing responses.
By defining a response_model, you protect your API output from leaking unwanted fields.
1. Basic Response Model
Define a Pydantic model and use it in your endpoint:
from fastapi import FastAPI
from pydantic import BaseModel
class Item(BaseModel):
name: str
price: float
app = FastAPI()
@app.get("/item", response_model=Item)
async def get_item():
return {"name": "Laptop", "price": 45000, "extra": "ignored!"}
Note: The field "extra": "ignored!" is removed in the final output.
2. Controlling Response Fields
Use response_model_exclude and response_model_include when needed.
from fastapi import FastAPI
from pydantic import BaseModel
class User(BaseModel):
username: str
email: str
is_admin: bool
app = FastAPI()
@app.get("/public-user", response_model=User, response_model_exclude={"is_admin"})
async def public_user():
return {"username": "john", "email": "john@example.com", "is_admin": True}
3. List Response Models
FastAPI handles lists natively in response models.
from fastapi import FastAPI
from pydantic import BaseModel
class Product(BaseModel):
name: str
price: float
app = FastAPI()
@app.get("/products", response_model=list[Product])
async def get_products():
return [
{"name": "Keyboard", "price": 900},
{"name": "Mouse", "price": 700},
]
4. Nested Response Models
from fastapi import FastAPI
from pydantic import BaseModel
class Address(BaseModel):
city: str
country: str
class Customer(BaseModel):
name: str
address: Address
app = FastAPI()
@app.get("/customer", response_model=Customer)
async def get_customer():
return {
"name": "Raghav",
"address": {"city": "Chennai", "country": "India"},
}
5. Response Model with Aliases
Use Pydantic aliases to format API output fields.
from fastapi import FastAPI
from pydantic import BaseModel, Field
class Book(BaseModel):
title: str = Field(..., alias="bookTitle")
author: str = Field(..., alias="bookAuthor")
app = FastAPI()
@app.get("/book", response_model=Book)
async def get_book():
return {"bookTitle": "FastAPI Guide", "bookAuthor": "John Doe"}
