It's very nice to see a small type checker in Python, for Python! This became much easier in the last 10 years, since the MyPy team basically "upstreamed" the typed_ast library they were using into the stdlib.

I found that there are not enough good teaching materials on type checkers -- e.g. the second edition of the Dragon Book lacks a type checker, which is a glaring hole IMO - https://news.ycombinator.com/item?id=38270753

Also, teaching material tends to have a bias toward type inference and the Hindley-Milner algorithm, which are NOT used by the most commonly used languages

So I appreciate this, but one thing in this code that I find (arguably) confusing is the use of visitors. e.g. for this part, I had to go look up what this method does in Python:

    # Default so every expr returns a Type.
    def generic_visit(self, node):
        super().generic_visit(node)
        if isinstance(node, ast.expr):
            return ANY

Also, the main() calls visit(), but the visitor methods ALSO call visit(), which obscures the control flow IMO. Personally, if I need to use a visitor, I like there to just be a single pass

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In contrast, Essentials of Compilation was released 1 or 2 years ago, in Racket and in Python. And the Python version uses the same typed AST module.

https://www.amazon.com/Essentials-Compilation-Incremental-Ap...

But it uses a more traditional functional style, rather than the OO visitor style:

https://github.com/IUCompilerCourse/python-student-support-c...

So one thing I did was to ask an LLM to translate this code from OO to functional style :-) But I didn't get around to testing it

(I looked at this code a week ago when it appeared on lobste.rs [1], and sent a trivial PR [2])

[1] https://lobste.rs/s/opwycf/baby_s_first_type_checker

[2] https://github.com/AZHenley/babytypechecker/pull/1