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Comment by woodruffw | Hacker News Reader

Yep, someone brought this up on another discussion forum. The post was intended to be explicitly about accomplishing the ser/de half as well, hence the emphasis on Pydantic :-)

(Python’s annotated types are very powerful, and you can do this and more with them if you don’t immediately need ser/de! But they also have limitations, e.g. I believe Union wasn’t allowed in isinstance checks or matching until a recent version.)

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If you're looking for serialization/deserialization, you might consider confactory [1]. I created to be a factory for objects defined in configs. It actually builds the Python objects without much effort on the user. It simply makes use of type annotations (though you can define your own serializers and deserializers).

It also supports complex structures like union types, lists, etc. I used it to create cresset [2], a package that allows building Pytorch models directly from config files.

[1]: https://pypi.org/project/confactory/ [2]: https://pypi.org/project/cresset/

I think it’s quite useful to separate ser/de, structural validation, and semantic validation. This is where I struggle with a library like ruamel.yaml, running deserialization and structural validation together, or Pydantic, running structural and semantic validation together. It’s not hard to write a Python type annotation for what you get from json.loads, and it’s also not hard to write a recursive function with a 200 line match statement that reflects on type annotations to convert that to typeddicts, data classes, and so forth. But semantic validation is a whole other problem, one that tends to be so domain specific it’s better deferred. Not that you shouldn’t do it, but that it belongs in its own data processing layer. Also this lets you be specific about what’s wrong with a piece of input. Bad JSON? A list where a dictionary was expected? An end timestamp that’s before the start? Sure, check each of these, and in context make invalid state unrepresentable, but invalid state after json.loads is very different from invalid state after validating your timestamps.

Yeah and in practise most people will probably be using Pydantic anyway. Just wanted to point out it's not strictly necessary. :)

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