pydantic VS mypy

Pydantic Documentation

Found in issue where maintainer answered that they are only going to support JSON

https://github.com/pydantic/pydantic/discussions/4097

The solution? Leverage Pydantic, a Python library that enables runtime data validation using type annotations.

Pydantic author here. We have plans for an improvement to pydantic where JSON is parsed iteratively, which will make way for reading a file as we parse it. Details in https://github.com/pydantic/pydantic/issues/10032.

Our JSON parser, jiter (https://github.com/pydantic/jiter) already supports iterative parsing, so it's "just" a matter of solving the lifetimes in pydantic-core to validate as we parse.

This should make pydantic around 3x faster at parsing JSON and significantly reduce the memory overhead.

Across this five-post series, we’ve journeyed from Pydantic’s basics—type validation and nested models—to advanced integrations with FastAPI, SQLAlchemy, and scalable techniques. You’ve learned how to build declarative, type-safe models, handle complex APIs, and optimize performance. To deepen your knowledge, explore the Pydantic documentation, contribute to the open-source project, or experiment with real-world use cases. Check out our GitHub repo for code samples and a Pydantic cheat sheet. Thank you for joining us—happy coding!

To dive deeper, check out the Pydantic documentation for advanced features like nested models, custom validators, and settings management. Happy coding!

Pydantic is bundled with FastAPI and is excellent for modelling, validating, and serialising API responses.

Pydantic Settings is a python library that extends 🚀 Pydantic for dealing with settings management.

- "Final names and attributes" https://github.com/python/mypy/pull/5522

Hashable cannot be used for an iterator even it's immutable according to the issue.

I reported the strange behaviour as the issue.

is used with the type checkers such as mypy, pyright, pyre-check, pytype, etc.

It's not so bad here, but with a big enough language, and more complicated algorithms, it gets unwieldy.

That said, I agreed here that visitors are useful when you need to say traverse all string literals in an AST, at arbitrary depths: https://lobste.rs/s/jdgjjt/visitor_pattern_considered_pointl...

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A sign that this issue isn't settled is that two of the more complex type checkers make opposite decisions

- MyPy uses visitors extensively - https://github.com/python/mypy/tree/master/mypy

- TypeScript mostly uses switch/case functions - https://github.com/microsoft/TypeScript/blob/main/src/compil...

I'd be interested in analysis of why that is, but I suspect it's mainly style

https://github.com/python/mypy/blob/501a07b45af8e44eda665e53...

Also did you know mypy ignores typing of class decorators? You simply can't return a different type other than type[thisclass].

To expand on this, you could be maintaining a list of Cats, call a function which accepts a list of Animals, and that function could append a Dog without causing a type error.

https://github.com/python/mypy/issues/4976 is an interesting discussion on this in Python: should a TypedDict be allowed where a dict argument is accepted? The basic consensus is no, because then one could modify the dict, adding keys that wouldn't be allowed in the specific TypedDict.

But this means that often you have to do cast(dict, my_typed_dict) to be able to interop with third party libraries that (quite naturally) believed that by typing their arguments as accepting a dict, they'd be accepting all dict-like things.

Mypy:

Hynek Schlawack, a well-known Pythonista and open source contributor, has recommended not to run mypy as part of your pre-commit workflows. And Shantanu Jain, a CPython core developer and mypy maintainer, has an excellent write-up of some of the gotchas that come with running mypy as a pre-commit hook. By default, mypy runs in an isolated environment, meaning it won't have access to your project's virtual environment and therefore won't be able to fully analyze type hints when your code makes use of dependencies. Additionally, pre-commit hooks only pass the changed/staged files by default, whereas mypy needs to see the entire repository to function correctly. I've also noticed nesting your code under other folders (like my backend/ directory) can also cause problems.