[Spivak]

But TypeScript erases (its) types at runtime, exactly like Python. Python is Python's TypeScript. Whether you want TS or JS-like semantics is entirely dependent on whether you use a type checker and whether you consider its errors a build breaker.

[scuff3d]

I'm not sure what you're trying to say here. If you mean Python's type annotations are erased at runtime... Okay? It still has runtime type information. It's not "erasure" as that term applies to Java for example. And Typescript compiles down to JavaScript, so obviously it's runtime behavior is going to be the same as JavaScript.

In my view it's always a mistake to try and tac static typing on top of a dynamic one. I think TS's approach is better than Python's, but still not nearly as good as just using a statically typed language.

[ngrilly]

The fact that the types are reflected at runtime is what makes FastAPI/Pydantic possible, letting us use Python types to define data models used for serialization, validation, and generating. In TypeScript, we have to use something like Zod, instead of "normal" TypeScript types, because the types are not reflected at runtime.

[scuff3d]

I think a couple of things have to be untangled here.

The problem we are talking about in both Python and TS comes from the fact that they are (or compile down to) dynamic languages. These aren't issues in statically typed languages... because the code just won't compile it it's wrong and you don't have to worry about getting data from an untyped library.

I don't know a lot about Zod, but I believe the problem you are referring to is more about JavaScript then TS. JavaScript does a LOT of funky stuff at runtime, Python thank God actually enforces some sane type rules at runtime.

My point was not about how these two function at runtime. My point was that if you want to tac static typing onto a dynamic language, Typescripts approach is the better one, but even if can't fix the underlying issues with JS.

You could take a similar approach in Python. We could make a language called Tython, that is statically typed and then compiles down to Python. You eliminate an entire class of bugs at compile time, get a far more reliable experience then the current weirdness with gradual typing and linters, and you still get Pythons runtime type information to deal with things like interopt with existing Python code.

[AlienRobot]

Typescript requires a compiler to produce valid Javascript. Python 3 shoved types into Python 3 without breaking backwards compatibility I think.

You would never have typing.TYPE_CHECKING to check if type checking is being done in TypeScript, for example, because type hints can't break Javascript code, something that can happen in Python when you have cyclic imports just to add types.

[ehutch79]

Not it doesn’t. It doesn’t throw errors, but they’re still introspectable in python, unlike typescript