You start talking about GIL and then you talk about non-atomic data manipulation, which happen to be completely different things.
The only code that is going to break because of "No GIL" are C extensions and for very obvious reasons: You can now call into C code from multiple threads, which wasn't possible before, but is now. Python code could always be called from multiple python threads even in the presence of the GIL in python.
The only code that is going to break because of "No GIL" are C extensions and for very obvious reasons: You can now call into C code from multiple threads, which wasn't possible before, but is now. Python code could always be called from multiple python threads even in the presence of the GIL in python.