On C$50 Finance, when I refresh my web server too many times, AlphaVantage stops returning stock quotes.

As a result, Flask returns "TypeError: 'NoneType' object is not subscriptable" when my code tries to use any variable created with the lookup() function.

I realized AlphaVantage was the culprit, when I found this answer on CS50 Stack Exchange.

CS50 Exchange Related Question: pset7 index TypeError: 'NoneType' object is not subscriptable

I called the API directly from my browser's address bar. AlphaVantage returned an error message instead of an excel download.

API call pasted into browser and refreshed a couple times: "https://www.alphavantage.co/query?apikey=NAJXWIA8D6VN6A3K&datatype=csv&function=TIME_SERIES_INTRADAY&interval=1min&symbol=GOOG"

AlphaVantage Error: "Please consider optimizing your API call frequency"

AlphaVantage begins to respond again, given enough time. However, for debugging purposes, I need to refresh my app many times. It is inefficient to wait for AlpaVantage to work again, every time I need to try new code.

How can I replace AlphaVantage to debugging purposes?

There is a long list of APIs here, but all of them either seem to be paid, or much more confusing https://www.programmableweb.com/news/96-stocks-apis-bloomberg-nasdaq-and-etrade/2013/05/22