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Comment by JackSlateur | Hacker News Reader

This is true, there is a performance cost

Apart from what has been said, I find pydantic interesting even in the middle of my code: it can be seen as an overpowered assert

It helps making sure that the complex data structure returned by that method is valid (for instance)

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Yeah, I'd agree with that. Validation rules are like an extension to the type system. Invariants are useful at the edges of a system but also in the core. If, for example, I can be told that a list is non-empty then I can write more clean code to handle it.

In Java they got around the external-dependency-in-the-core-model problem by making the JSR-380 specification that could (even if only in theory) have multiple implementations.

In clojure you don't need to worry about another external dependency because the spec library is built-in. One could argue that it's still a dependency even if it's coming from the standard library. At the point I'd say, why are we worried about this? it's to isolate our core from unnecessary reasons to change.

I get that principled terms it's not right but, if those libraries change on API on a similar cadence to the programming language syntax, then it doesn't impact in practical terms. It's these kind of pragmatic compromises that distinguish Python from Java - after all, 'worse is better'.

You could also use a TypedDict for that, though?

PEP 764[0] will make them extra convenient.

[0]: https://peps.python.org/pep-0764/

Typing is declarative

In the end, it ensures nothing and accepts everything

  $ cat test.py
  from typing import TypedDict
  class MyDict(TypedDict):
        some_bool: bool
  print(MyDict(some_bool="test", unknown_field="blabla"))

   $ ./test.py
   {'some_bool': 'test', 'unknown_field': 'blabla'}

Rolls eyes… Of course you need to use a type checker. Who doesn't these days?

Of course, I need runtime validation

Sure, runtime validation is useful – at the boundaries of your domain! After that your type checker should ensure your data has the shape your code expects.

In other words: Parse, don't validate. https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...

Yes

The boundaries of my domain is everywhere, because stuff are built everywhere. I'm sure I am a bad dev, yet prebuilding all possibles objects I might need everytime I receive an input sounds stupid at best

(btw, this post sounds a lot like "if it compiles, it works", not gonna do this, especially in Python)

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