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Python / FastAPI Basics Interview Questions

What is the response_model parameter in FastAPI and why should you use it?

The response_model parameter on a route decorator tells FastAPI which Pydantic model to use for filtering and serialising the response. Even if the endpoint returns more data internally, only the fields defined in the response model are included in the JSON output.

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class UserIn(BaseModel):
    username: str
    password: str
    email: str

class UserOut(BaseModel):
    username: str
    email: str
    # password intentionally excluded

# response_model filters the output — password never appears in response
@app.post("/users", response_model=UserOut)
def create_user(user: UserIn) -> UserOut:
    # Even if we return the full user object, password is stripped
    return user  # FastAPI applies UserOut filtering

# response_model_exclude_unset: only include fields explicitly set by caller
@app.get("/items/{id}", response_model=UserOut, response_model_exclude_unset=True)
def get_item(id: int):
    return {"username": "alice", "email": "a@b.com"}

# List response
@app.get("/users", response_model=list[UserOut])
def list_users():
    return [{"username": "alice", "email": "a@b.com", "password": "secret"}]

response_model options
OptionEffect
response_model=UserOutFilters output to only UserOut fields
response_model_exclude_unset=TrueOmits fields not explicitly set (no default-value noise)
response_model_exclude_none=TrueOmits fields that are None
response_model_include={'field'}Only include specific fields
response_model_exclude={'field'}Exclude specific fields
What is the primary security benefit of using response_model in FastAPI?
If response_model_exclude_unset=True is set and a field has a default value but was not explicitly set by the handler, what happens?

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