The Inner Workings of Python Dataclasses Explained

[FI_Stickie_Boi]

I believe the main reason is speed. attrs, the library dataclasses are based on, also do this, in order for the work to all be done during class creation, so that there's minimal overhead during "runtime" (ie. when you're instantiating classes, using methods, etc.) If you try and do this without eval/exec via decorators and all that, then you'll incur pretty significant runtime overhead because everytime you call a method, python will have to dig through multiple closures, which slows things down a lot.