Cannot import a module from a package

root_package
|---main_package
|    |--__init__.py
|    |--main.py 
|---write_to_file
|    |--__init__.py
|    |--write.py  
|
|---__init__.py

I'm tryin to access a function that's in main.py from write.py like this:

write.py

from root_package.main_package.main import getProduct

and I get the following error: No module named 'root_package'

I know this is basic and most asked question, but I can't find the answer from other posts. I'm just not seeing what is wrong. Thanks

EDIT: I'm trying to run write.py script like this:

root_package\write_to_file\write.py python write.py