Query parameters — the part after the ? in a URL. In FastAPI, any function parameter that isn't in the path is automatically treated as a query parameter:

@app.get("/items")
def list_items(skip: int = 0, limit: int = 10):
    items = ["a", "b", "c", "d", "e", "f"]
    return items[skip : skip + limit]

GET /items?skip=2&limit=3 returns ["c", "d", "e"]. The defaults mean GET /items works too — it returns the first 10 items.

Exactly. If a parameter has a default, it's optional. If it doesn't, it's required:

@app.get("/search")
def search(q: str, max_results: int = 5):
    return {"query": q, "max_results": max_results}

Here q is required — calling GET /search without ?q=something returns a 422 error. But max_results has a default of 5, so it's optional.

Use None as the default:

from typing import Optional

@app.get("/tasks")
def list_tasks(
    status: Optional[str] = None,
    priority: Optional[int] = None,
    limit: int = 20
):
    result = {"limit": limit, "filters": {}}
    if status is not None:
        result["filters"]["status"] = status
    if priority is not None:
        result["filters"]["priority"] = priority
    return result

GET /tasks returns all tasks. GET /tasks?status=done filters by status. GET /tasks?status=done&priority=1 filters by both. The function checks what was provided and builds the response accordingly.

Just use both. Path parameters go in the URL path, query parameters are the rest:

@app.get("/users/{user_id}/tasks")
def user_tasks(
    user_id: int,
    status: Optional[str] = None,
    skip: int = 0,
    limit: int = 10
):
    return {
        "user_id": user_id,
        "status_filter": status,
        "skip": skip,
        "limit": limit
    }

FastAPI knows user_id is a path parameter because it appears in "/users/{user_id}/tasks". The rest are query parameters. Clean, typed, and validated — all from a function signature.