Well. I think you mean proprietary product. The restriction is in the license you use, not on whether the code is visible or not. In any case, it is assumed that you want to hide the code from the user.
There is a way to partially hide the source code, and only used the byte compiled code, which nevertheless can be imported and used.
Imported modules can be supplied only in .pyc byte compiled format. They can be imported by a main app in Python. Note that pyc files are not portable between python versions.
Take a look at utilities which embed modules and a python interpreter together into an executable, like PyInstaller and py2exe.