I worked in a large python codebase with no types and no tests. It was.... barely ok. Honestly I found that the biggest problem with no types was less specific

I worked in a large python codebase with no types and no tests. It was.... barely ok. Honestly I found that the biggest problem with no types was less specifically about "oops you passed a dict in but it was expecting a string haha runtime error" and more about: [line 2359 of somefile.py, 20 calls deep]: def snafucate(id, f, connection): [me]: ok connection is the same db connection we've been using. id is the UUID. What is f though? (clicks find-usages and picks the probably correct call site) [that call site]: def snafucate_caller(f, connection): [me]: [repeats this process until I deduce that f is at various points in the codebase at various call sites and sibling methods: a file IO object, a filename str, a custom type that represents a file path, a 2-tuple of (filename str, file IO object), and in one code path that I think it's actually unused, a dict that's never populated with anything. And just lots of crap like this. It's some awful combination of poor rigor about naming, not consistently using the same schemes for args (do you usually take in IO objects, strings representing paths, or t.Union[IO,str]?), no testing, poor documentation, no type hinting, and of course the good ole' "this code has had 12 owners across 20 years, ranging from well-meaning new grads, to "experienced" people who are bad at coding, to ninjas who are too terribly clever for the rest of us to hope to keep up". And yet somehow we still got stuff done in it. And one of the things I tried to do when working in it was either adding type hints once I figured them out, or if they were too arcane (like Union[Dict[Any,Any],str,IO,Tuple[str, IO]]) I would try to squash them down enough to make the type sane. As I was slowly dragged through the codebase, the types slowly started peppering in, and it got easier and easier, especially as I got promoted and had some juniors under my wing who I could ask to help do the same gradual-typing-as-you-spelunk process. I kind of miss it tbh, one of the things I was doing right before I left was toying with proposing an official set of domain-specific type hints and type aliases that would've made the whole codebase a lot easier to follow if people were willing to join me in my quest.