Guide: Migrating Python Projects from mypy to ty and Pyrefly

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A practical migration guide documents how the fastkml Python package was moved from mypy to Astral's ty and Meta's pyrefly type checkers. The guide recommends running both tools simultaneously, as they each catch different subsets of type errors that the other may miss. Before making any changes, developers are advised to establish a baseline error count and categorize issues by error kind to identify systemic root causes rather than fixing problems file by file. The guide also warns that mypy configuration flags do not map one-to-one to ty or pyrefly equivalents, urging developers to re-derive intent rather than hunt for identical settings. Finally, it stresses verifying that both tools pass cleanly alongside a full test suite before retiring the existing mypy configuration.

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