In Python 3.6, the docs for ThreadPoolExecutor mentions the following line:

initializer is an optional callable that is called at the start of each worker thread; initargs is a tuple of arguments passed to the initializer. Should initializer raise an exception, all currently pending jobs will raise a BrokenThreadPool, as well any attempt to submit more jobs to the pool.

But, from my experiment in Python 3.6 and 3.7, I have observed that `initializer` and `initargs` were introduced in Python 3.7.