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Python / FastAPI Basics Interview Questions

How do you read HTTP headers and cookies in FastAPI?

FastAPI provides Header() and Cookie() parameter types. Headers are automatically converted from HTTP's hyphen-case to Python's snake_case. You can also set cookies on responses.

from fastapi import FastAPI, Header, Cookie, Response
from typing import Annotated

app = FastAPI()

# Read request headers
@app.get("/headers")
def read_headers(
    user_agent: Annotated[str | None, Header()] = None,
    accept_language: Annotated[str | None, Header()] = None,
    x_api_key: Annotated[str | None, Header()] = None,  # X-Api-Key header
):
    # FastAPI converts Accept-Language → accept_language automatically
    return {
        "user_agent":      user_agent,
        "accept_language": accept_language,
        "x_api_key":       x_api_key,
    }

# Duplicate headers → list
@app.get("/multi-header")
def multi_header(
    x_token: Annotated[list[str] | None, Header()] = None,
):
    return {"x_token": x_token}  # all values of X-Token header

# Read cookies
@app.get("/profile")
def read_profile(
    session_id: Annotated[str | None, Cookie()] = None,
):
    return {"session_id": session_id}

# Set a cookie in the response
@app.post("/login")
def login(response: Response, username: str):
    response.set_cookie(
        key="session_id",
        value=f"session_{username}",
        httponly=True,   # not accessible via JS
        secure=True,     # HTTPS only
        samesite="lax",  # CSRF protection
        max_age=3600,    # 1 hour
    )
    return {"message": "Logged in"}

# Delete a cookie
@app.post("/logout")
def logout(response: Response):
    response.delete_cookie("session_id")
    return {"message": "Logged out"}

How does FastAPI convert HTTP header names to Python parameter names?
What does httponly=True on a response cookie do?

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