python 45 lines · 8 steps

How PATCH partial updates work in FastAPI

A FastAPI route that updates only the fields a client actually sends, using Pydantic's exclude_unset to distinguish omitted from null.

Explained by highlit
FastAPI Intermediate partial-update validation dependency-injection orm rest
1from typing import Optional
2 
3from fastapi import APIRouter, Depends, HTTPException, status
4from pydantic import BaseModel, EmailStr, Field
5from sqlalchemy.orm import Session
6 
7from .database import get_db
8from .models import User
9 
10router = APIRouter(prefix="/users", tags=["users"])
11 
12 
13class UserUpdate(BaseModel):
14 full_name: Optional[str] = Field(default=None, max_length=120)
15 email: Optional[EmailStr] = None
16 bio: Optional[str] = Field(default=None, max_length=500)
17 is_active: Optional[bool] = None
18 
19 
20class UserOut(BaseModel):
21 id: int
22 full_name: str
23 email: EmailStr
24 bio: Optional[str]
25 is_active: bool
26 
27 model_config = {"from_attributes": True}
28 
29 
30@router.patch("/{user_id}", response_model=UserOut)
31def patch_user(user_id: int, payload: UserUpdate, db: Session = Depends(get_db)):
32 user = db.get(User, user_id)
33 if user is None:
34 raise HTTPException(status.HTTP_404_NOT_FOUND, detail="User not found")
35 
36 changes = payload.model_dump(exclude_unset=True)
37 if not changes:
38 raise HTTPException(status.HTTP_400_BAD_REQUEST, detail="No fields to update")
39 
40 for field, value in changes.items():
41 setattr(user, field, value)
42 
43 db.commit()
44 db.refresh(user)
45 return user
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1exclude_unset separates fields a client omitted from fields set explicitly to null, which is what makes PATCH semantics correct.
  2. 2Optional fields with defaults let one schema validate any subset of updatable attributes.
  3. 3Injecting the session with Depends keeps the handler testable and ties the connection to the request lifecycle.

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