python 52 lines · 8 steps

Building a signup endpoint in FastAPI

A user-registration route that validates input, checks for duplicates, persists to the database, and sends a welcome email in the background.

Explained by highlit

FastAPI Intermediate dependency-injection request-validation background-tasks orm rest-api

1from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException, status
2from pydantic import BaseModel, EmailStr
3from sqlalchemy.orm import Session
4 
5from .database import get_db
6from .models import User
7from .services.mailer import send_email
8 
9router = APIRouter(prefix="/users", tags=["users"])
10 
11 
12class SignupRequest(BaseModel):
13    email: EmailStr
14    full_name: str
15 
16 
17class SignupResponse(BaseModel):
18    id: int
19    email: EmailStr
20 
21    class Config:
22        from_attributes = True
23 
24 
25def send_welcome_email(email: str, full_name: str) -> None:
26    send_email(
27        to=email,
28        subject="Welcome aboard!",
29        template="welcome",
30        context={"name": full_name},
31    )
32 
33 
34@router.post("", response_model=SignupResponse, status_code=status.HTTP_201_CREATED)
35def signup(
36    payload: SignupRequest,
37    background_tasks: BackgroundTasks,
38    db: Session = Depends(get_db),
39) -> User:
40    if db.query(User).filter(User.email == payload.email).first():
41        raise HTTPException(
42            status_code=status.HTTP_409_CONFLICT,
43            detail="A user with this email already exists.",
44        )
45 
46    user = User(email=payload.email, full_name=payload.full_name)
47    db.add(user)
48    db.commit()
49    db.refresh(user)
50 
51    background_tasks.add_task(send_welcome_email, user.email, user.full_name)
52    return user

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Pydantic models validate incoming JSON and shape outgoing responses, keeping the endpoint body free of manual parsing.
2Depends lets FastAPI inject a per-request database session so the handler never manages its own connection lifecycle.
3BackgroundTasks defers slow work like email until after the response is sent, keeping the request fast.

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