PyCharm can't find import in same folder

It is better if you replace that with:

from pca_mixtures.constants import *

The issue is because of how PyCharm sets PYTHONPATH to the root of your project.

While you can add directories as Sources, I would not recommended. That will make your project fragile.

You can right click pca_mixtures and then mark the directory as Sources Root, this should solve your problem. Good luck!