Python Mutable References with Caching
So, while working with caching and scrapping, I understood the difference between immutable and mutable objects/datatypes very clearly. I had a scenario, where I am webscraping an API, the code looks like this.
from aiocache import cached
@cached(ttl=7200)
async def get_forecast(station_id: str) -> list[dict]:
data: dict = await scrape_weather(station_id)
# doing some operation
return forecasts
and then using this utility tool in the endpoint.
async def get_forecast_by_city(
param: Annotated[StationIDQuery, Query()],
) -> list[UpcomingForecast]:
forecasts_dict: list[dict] = await get_forecast(param.station_id)
forecasts_dict.reversed()
forecasts: deque[UpcomingForecast] = deque([])
for forecast in forecasts_dict:
date_delta: int = (
date.fromisoformat(forecast["forecast_date"]) - date.today()
).days
if date_delta <= 0:
break
forecasts.appendleft(UpcomingForecast.model_validate(forecast))
return list(forecasts)
But, here is the gotcha, something I was doing inherently wrong. Lists in python are mutable objects. So, reversing the list modifies the list in place, without creating a new reference of the list. My initial approach was to do this