What is FastAPI response model and how to use it?

Question

Accepted Answer

**response_model** controls **what data is returned** in API responses.

```python
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
from typing import Optional

app = FastAPI()

# Models
class UserCreate(BaseModel):
    name: str
    email: EmailStr
    password: str  # Should not be in response!

class UserResponse(BaseModel):
    id: int
    name: str
    email: EmailStr
    is_active: bool

class Config:
        orm_mode = True

# Use response_model to filter output
@app.post("/users/", response_model=UserResponse)
async def create_user(user: UserCreate):
    db_user = create_user_in_db(user)
    return db_user  # Only UserResponse fields returned

# List response
@app.get("/users/", response_model=list[UserResponse])
async def get_users():
    return User.objects.all()

# Filtering response
@app.get("/users/{user_id}", response_model=UserResponse)
async def get_user(user_id: int):
    user = get_user_or_404(user_id)
    return user

# Exclude_unset — only return set fields
@app.put("/users/{user_id}", response_model=UserResponse)
async def update_user(
    user_id: int,
    user: UserCreate,
):
    updated = update_user_in_db(user_id, user)
    return updated
```

**Benefits:**
- Hide sensitive fields (password).
- Automatic response validation.
- OpenAPI documentation.

Python: What is FastAPI response model and how to use it?