How to get a Router (Motem) Local IP Address for any OS using Python Sockets

So I simply want to know how to get the ROUTER or MOTEM's Local IP Address using Sockets in Python; this will be used to transfer files using LAN. So I know this exists socket.gethostbyname(socket.gethostname()). But, that only really works on Windows, is there a universal or almost universal command in sockets or some other module to get the LAN or Network IP Address? This is an issue I've had for a while so any help would be appreciated. I am currently on Windows but if I try to do this command on Mac or Linux it will not work, so could there maybe be a way of like if you are on windows then do this, else if you are on Mac do this, else if Linux then do something different, else default to the window solution. Something like that. Maybe I just need an explanation of this.