"You should use context instead of global variables to pass variable values between the various methods in your algorithm." (this is from this page: https://www.quantopian.com/docs/user-guide/tools/algo-api#attaching-pipelines under the 'Initializing State' sub-heading')
I don't know If this is the right place to post this, and sorry for the simple/dumb question, but I was just interested to know why we can't just define global variables instead of using context?
Thanks
Placing 'global' variables in the context object (as a custom attribute) is the recommended practice but not currently required.
The reason stems from a 'persistence' feature in Zipline (the open source software that runs Quantopian backtests). This feature was intended to store an algorithm's state so it could be stopped and re-started. Or, similarly for live trading, could allow an algo to stop when markets weren't open and then restart before trading. The mechanism relied on saving the context object, and its data, to disk. No other data was stored. Therefore, storing data in globals was discouraged because those globals would not 'persist' if an algo was restarted.
However, Quantopian never implemented 'persistence' so defining globals outside of the context object currently works fine. The risk is this may be implemented in the future. Note that some other platforms which rely on Zipline do implement this. One such implementation is pylivetrader which can run Quantopian algos for live trading with Alpaca. There one must put all state and global variables in the context object to allow for a 'live' algo being shutdown and restarted.
Hope that explanations helps.
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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.
