Your analysis is right - your algo just sells Apple short, indiscriminately, over and over. Since Apple has been going down for the last few months, you made a lot of (fictional) money.
As it is today, Quantopian sets very few rules.
- Commissions are included by default, but you can turn them off.
- Slippage is included by default, but you can turn that off too.
- You can't artificially increase your cash, or magically create new stock, or other deus ex machina
- You can't get around look-ahead bias - there's no real way for you to include future data in your algorithm.
With such a short set of rules, the rest of the algorithm writing is left to best practice. If you clone the sample agorithm you'll see some of those best practices implemented. The sample algorithm includes a max- and min-notional. If you take the code from your sample and apply some notional limits, you'll get a more reasonable-looking result.
I agree that we need to make it easier to avoid crazy situations like this. Cash management, notional limits, margin limits, borrowing limits, etc. are all ripe for improvements. I'm not sure when we'll get to it, but they are on the list.
The material on this website is provided for informational purposes only and does not constitute an offer to sell, a solicitation to buy, or a recommendation or endorsement for any security or strategy, nor does it constitute an offer to provide investment advisory services by Quantopian. In addition, the material offers no opinion with respect to the suitability of any security or specific investment. No information contained herein should be regarded as a suggestion to engage in or refrain from any investment-related course of action as none of Quantopian nor any of its affiliates is undertaking to provide investment advice, act as an adviser to any plan or entity subject to the Employee Retirement Income Security Act of 1974, as amended, individual retirement account or individual retirement annuity, or give advice in a fiduciary capacity with respect to the materials presented herein. If you are an individual retirement or other investor, contact your financial advisor or other fiduciary unrelated to Quantopian about whether any given investment idea, strategy, product or service described herein may be appropriate for your circumstances. All investments involve risk, including loss of principal. Quantopian makes no guarantees as to the accuracy or completeness of the views expressed in the website. The views are subject to change, and may have become unreliable for various reasons, including changes in market conditions or economic circumstances.