Is it possible to write an API with Python so you can connect a physical ON and OFF switch via USB to a PC and when user presses the switch to ON or OFF, the python program detects it and send a signal to a web app and shows ON or OFF message on the website?
I am sorry if what I am asking its not clear enough!
Yes, it is possible. Reading USB devices can be done with Python. In linux USB device inputs can be found in some files(e.g. /dev/ttyUSB0). By reading those files you can get the information that you need. Putting here link that will be helpful
Firstly, you can't write an API to interact with hardware in python. You would have to use the pre-existing windows API(or the API provided by the Operating system that you are using) in order to interact with hardware in such a high-level language. If you want to interact with hardware in python, and detect switch presses, releases etc, I would recommend you used a microcontroller such as a raspberry pi(for python) or an arduino(for C++). The respberry pi provides a very easy way to interact with hardware in python. If you still want to interact with a USB stick in python(but not acting as a switch) you can use the pyusb library.
