python 41 lines · 7 steps

Building a users router in FastAPI

A FastAPI router pairs Pydantic response models with SQLAlchemy queries to serve validated, shape-controlled JSON.

Explained by highlit
FastAPI Intermediate routing serialization dependency-injection orm validation
1from datetime import datetime
2from fastapi import APIRouter, Depends, HTTPException, status
3from pydantic import BaseModel, ConfigDict, EmailStr
4from sqlalchemy.orm import Session
5 
6from .database import get_session
7from .models import User
8 
9router = APIRouter(prefix="/users", tags=["users"])
10 
11 
12class ProfileOut(BaseModel):
13 model_config = ConfigDict(from_attributes=True)
14 
15 id: int
16 username: str
17 email: EmailStr
18 full_name: str | None = None
19 is_verified: bool
20 created_at: datetime
21 
22 
23class UserListItem(BaseModel):
24 model_config = ConfigDict(from_attributes=True)
25 
26 id: int
27 username: str
28 is_verified: bool
29 
30 
31@router.get("", response_model=list[UserListItem])
32def list_users(session: Session = Depends(get_session)):
33 return session.query(User).order_by(User.username).all()
34 
35 
36@router.get("/{user_id}", response_model=ProfileOut)
37def get_user(user_id: int, session: Session = Depends(get_session)):
38 user = session.get(User, user_id)
39 if user is None:
40 raise HTTPException(status.HTTP_404_NOT_FOUND, detail="User not found")
41 return user
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STEP 01

Walkthrough

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Three takeaways
  1. 1Separate response models let each endpoint expose exactly the fields it should, hiding sensitive columns like email from list views.
  2. 2from_attributes lets Pydantic read ORM objects directly, so you can return SQLAlchemy models and FastAPI serializes them for you.
  3. 3Depends injects a database session per request, keeping route functions free of setup and teardown boilerplate.

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