Problem
I had a problem where I couldn't get Python to notice a package I just made.
What's great about Python is that you can work on a module, say my_file.py, and in another script write 'import my_file' to include that script in the current one. To include this module in a package, you merely add a blank __init__.py file to the folder the script is in. If the script is in a folder called 'my_folder', adding a __init__.py file to that folder will allow you to run:
from my_folder import my_script
However, Python only knows about this package if it's in the same directory. I was trying to run some tests recently by having a separate 'tests' folder and I kept on getting an error of ModuleNotFoundError: No module named my_folder.
The reason it can't find the package is because Python only knows about packages loaded into the site-packages section of your Python installation.
For example, I use a virtual environment on my mac which loads all my packages (such as pandas, scipy, etc) here:
/Users/my_username/.virtualenvs/virtualenv_name/lib/python3.6/site-packages
When you install with pip, the packages get loaded there.
If I decided to build the package, then it would go there too, but I don't really want to do that while I'm just doing some brief testing.
Solution
Add a .pth file to the site-packages/ directory.
It's fairly simple. Let's say my my_folder package is in the following my_folder directory (it's a common design to have a package in a folder of the same name).
/Users/my_username/Documents/my_folder/my_folder/
I would take this line and put it in a file called my_folder.pth (it can be named anything, though) and put that pth file in the site-packages/ directory. It might seem odd to just have a text file with that one line of a directory, but it works.
The file's location would be /Users/my_username/.virtualenvs/virtualenv_name/lib/python3.6/site-packages/my_folder.pth.
Now you can run from my_folder import my_script without any errors!
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