The handle_data function is indeed called automagically every minute. The backtester looks for a user function called handle_data and, if found, then executes that function every minute. It must be called handle_data for this to work.
The handle_data method works exactly like a function scheduled using schedule_function, but it is automatically called every minute without the need to explicitly schedule it. If you have code you wish to run every minute then simply put the code in handle_data. Note the code in handle_data will be run before any scheduled code run at the same minute.
There is no more time allotted to processing handle_data. There really isn't any reason to place code there except as a convenience to not scheduling it every minute. If you are not running code every minute then probably do not use it. Schedule all your functions instead. It's user preference though. It's just a convenience method.
Good question. Good luck.
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