> This is because PyUnicode_DecodeFSDefaultAndSize calls _PyInterpreterState_GET_UNSAFE(), which already documents the potential NULL return value. 

_PyInterpreterState_GET_UNSAFE() is preferred over other functions getting the interpreter for best performances. _PyInterpreterState_GET_UNSAFE() is the most efficient way to access the interpreter. That's why I was talking about the cost of additional checks (to detect API misuage) at runtime.

> Understood, happy for this to be closed. Aware that I was misusing the API :-)

I close the issue.