Daniel Arbuckle's Mastering Python

The add done callback function

The future object will remember that function and when the job function is done, the callback function will be called with the future object as its only parameter. The code in the callback function can then call the future's result method to get the return value or exception that the job produced.

The callback function will always be called in the controlling process, but it might not be called in the same thread as the main part of the program.

When we use add done callback, we need to be careful of thread synchronization issues, which is a big reason to prefer the wait or as_completed functions when possible. The future objects also have a cancel method.