I know absolutely nothing about Python, but the way the RunCommand plugin works is that it will return whatever is displayed by a program using STDOUT (console). So whatever program, Python or otherwise that you want to access should have the ability to output your desired result in STDOUT. An example of this are cmd.exe (DOS) commands, that output to a console window using STDOUT.
Any program you write should NOT create a GUI (graphical user interface) window, but should be designed and / or compiled to use a CUI (character user interface) output. Basically the program should run in a cmd.exe console window, which RunCommand can hide.
You will need to be careful that you match up the OutputType
option on the measure to the encoding that your program uses for output.
I would stay away from any purported Python plugins for Rainmeter at this time, I don't know of any that are currently supported and that I would really trust to work reliably.
I wouldn't be opposed in any way to an effort to integrate Python libraries with Rainmeter in a manner much as Lua is currently integrated. I think Python is a pretty good, very popular, and reasonably accessible language, and it has the additional charm of being a bit more robust and modern than Lua, for instance in the support for Unicode. This would not be a trivial effort, and I'm not suggesting it is in any way in the works, but I'd certainly be in favor of it.