Wait for ThreadpoolExecutor finish reading files to continue PYTHON

  multithreading, python, readfile

so I’m new to multi-threading in python.
I use thread pool executor to read big data files at the same time and combine them into one.
I want to delete duplicate lines but to do this I need to wait for the threads to be finished right?
I tried many different approaches finding here but no-one seems to work.
Appreciate any help!

Source: Python Questions