That sounds like the opposite direction: ctypes is for when you have a Python program which wants to call a C library, not when you want C code to call Python. As a concrete example, I work with JPEG 2000 imagery. I use ctypes so Python programs can load an image using a JP2 codec such as OpenJPEG to decode the image and save image tiles. Given the complexity of OpenJPEG, it would be a major project to port it to any other language and since it does a massive amount of computation the overhead of ctypes calling it is a small rounding error in the total processing time.

Unless the PyO3 documentation is completely leaving out a major application, it doesn't support this scenario.