Pygame - blitting预览图片

First off, this is a school assignment so I want to be upfront about that. Second I'm just asking for advice on the approach, possible help with the code. I'm working on a MSPAINT style clone using some pre-existing code from our book. The code already has the use of the draw.line when pressing mouse button 1. Teacher wants us to add ability to make circles or rectangles. I'm working on the circle part and I have figured out (thanks to the forums on here) how to implement what I wanted to do with the MOUSEBUTTONDOWN and MOUSEBUTTONUP events.. This has brought me to a new Question.. How would I blit then erase then blit a preview of the circle until it is the size the user wants and they release the MOUSEBUTTON and view the final blit... First off, this is a school assignment so I wan