A small type checker I made for Python as part of my blog post, Baby's first type checker.
It is ~350 lines of Python that type checks a single Python code file. It supports primitive types, containers, functions, assignments, binary operators, indexing, type unions, and a few scenarios of type narrowing.
To try it, run python3 babytypechecker.py examples/simple.py. It will report any type errors. For more diagnostics, use --trace to see all individual type checks (even the ones that pass).