Iterating a pure-Python list in one thread while modifying it in another will not give you true C-level undefined behaviour. It might throw an exception or retu

Iterating a pure-Python list in one thread while modifying it in another will not give you true C-level undefined behaviour. It might throw an exception or return the same element twice, etc., but it won't crash or do any nasal daemon stuff (time travelling crashes). In part, that's due to the Python GIL. What this articles says is that the buffer protocol allows bugs that are qualitatively different.