How dose python test discovery work?

I have a project setup with the following structure:

/
  script.py
  /tests
    __init__.py
    test_some_thing.py
    test_some_other_thing.py

in my script.py file I have the following line of code:

print('just some random text!!!!!!')

When I run:

python -m unittest discover

It runs the tests as expected but also outputs: just some random text!!!!!!

Can anyone explain exactly how test discovery works? I had assumed it would look for classes that were derived from unittest.TestCase and then all functions that are in there, but it appears to be running other code as well.