Use Automator to run Python script when files added to folder
Launch Automator, and choose 'Folder Action' as the document type. 
Then set the target folder in the drop-down menu at the top of the window. 

Add a "Run Shell Script" action. You have to choose one of the 'shell' environments in the drop-down menu, which include either the system python /usr/bin/python (which is v2, and EOLed); or a Unix shell, where you can call the python script externally, with something like:

/usr/local/bin/python3 &lt;path to python script&gt; "$1"


Alternatively to using the Run Shell Script action, you could use the AppleScript action with a do shell script command.

Save your workflow. The workflow is saved to ~/Library/Scripts/Folder Action Scripts

You can configure Folder Actions for any folder by right-clicking in the Finder and selecting "Folder Actions Setup".

(I have had intermittent success hacking the saved workflow in a text editor, and altering the 'shell' path to another environment. /usr/bin/swift works, but /usr/local/bin/python doesn't. More work required.)