But don’t you loose polymorphism?

No. You’ll have to be more specific about what kind of polymorphism you mean (it’s an overloaded term), but you can have type unions, like int | str.

Your points 1-3 are handled by running the code and reading the error messages, if any

Not unless you have ridiculously exhaustive tests, which you definitely don’t. And running tests is still slower than your editor telling you of your mistake immediately.

I probably didn’t explain 4-6 well enough if you haven’t actually ever used static types.

They make it easier to navigate because your IDE now understands your code and you can do things like “find all references”, and “go to definition”. With static types you can e.g. ctrl-click on mystruct.myfield and it will go straight to the definition of myfield.

They make the code easier to understand because knowing the types of variables tells you a lot of information about what they are and how to use them. You’ll often see in untyped code people add comments saying what type things are anyway.

Refactoring is easier because your IDE understands your code, so you can do things like renaming variables and moving code and it will update all the things it needs to correctly. Refactoring is also one of those areas where it tends to catch a lot of mistakes. E.g. if you change the type of something or the parameters of a function, it’s very easily to miss one place where it was used.

I don’t think “you need to learn it” really counts as slowing down development. It’s not that hard anyway.

I can understand the appeal for enterprise code but that kind of project seems doomed to go against the Zen of Python anyways, so it’s probably not the best language for that.

It’s probably best not to use Python for anything, but here we are.

I will grant that data science is probably one of the very few areas where you may not want to bother, since I would imagine most of your code is run exactly once. So that might explain why you don’t see it as worthwhile. For code that is long-lived it is very very obviously worth it.

Just in case that’s a genuine question, the reasons people like static types are:

1. Catch more bugs.
2. Catches bugs earlier (while you are writing the code). This is sometimes called “shift left”.
3. Fewer tests needed.
4. Code is easier to understand.
5. Code is easier to navigate.
6. Refactoring is much easier.
7. Development speed is faster (due to the above points).

Often people say it slows development down but it’s actually the opposite. Especially for large projects or ones involving multiple people.

The only downside really is that sometimes the types can get more complicated than they’re worth, but in that case you have an escape hatch via the Any type.

Pyright is very good. The rest are worthless though.

How powerful do you want it? Python’s type system is actually pretty good already and relatively sound when checked with Pyright (not Mypy though).

It’s not Typescript-level, but it’s better than e.g. Java or C++.

The main problem is Python developers writing code that can’t be statically type checked. E.g. using magically generate method names via __dict__ or whatever (I think lxml does that).