The thing is, though, that you mostly only get errors for code that is actually executed. So, your program is only fully type checked when all code paths are executed. In the case of python one can ameliorate this situation a bit by using mypy. At my job I see very often code being broken because, e.g., the signature of a function was changed but not in all places and so on. Now somebody will say that the IDE can solve that but these colleagues who are regularly breaking the code are actually using IDEs and it somehow still does not help. I have come to think that code that is not compiled and/or otherwise type checked is just not very serious and certainly not worthy of production environments.