Use Rs232, you can buy a USB-rs232 cable, with it you can hook up your pi directly on the PLC (assuming PLC has a RS232 port). I don't know much about python other than that it is not the fastest software
I had a Pi working as a MMI with Qt software. It worked extremely well and it was fast.
You can also wright a program in C using any text editor. This is propably the most efficient way.