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Python / FastAPI Interview Questions

What is the difference between path parameters and query parameters in FastAPI?

Path parameters are part of the URL path itself, declared with curly braces in the decorator and as function arguments. Query parameters are declared as function arguments that do not appear in the path string — FastAPI reads them from the URL query string automatically.

from fastapi import FastAPI
from enum import Enum

app = FastAPI()

# PATH parameter: part of the URL — /users/42
@app.get("/users/{user_id}")
def get_user(user_id: int):  # int type enforced automatically
    return {"user_id": user_id}

# QUERY parameters: after ? in URL — /items?skip=0&limit=10
@app.get("/items")
def list_items(
    skip: int = 0,          # optional, default 0
    limit: int = 10,        # optional, default 10
    search: str | None = None,  # fully optional
):
    return {"skip": skip, "limit": limit, "search": search}

# Enum path param — validates allowed values
class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet  = "resnet"

@app.get("/models/{model_name}")
def get_model(model_name: ModelName):
    return {"model": model_name}
Path vs Query Parameters
AspectPath paramQuery param
URL example/users/{id}/users?id=42
DeclarationIn path string + function argFunction arg only (not in path)
Required by default?Yes — URL won't match without itNo if it has a default value
Type validationYes — automatic via type hintYes — automatic via type hint
How does FastAPI know a function parameter is a query parameter vs a path parameter?
What happens if a client sends /users/abc when the path parameter is typed as int?

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