--- title: "Migrate From mypy To ty And pyrefly" url: https://daily.dev/posts/migrate-from-mypy-to-ty-and-pyrefly-qlfxaaw4n source_url: https://dev.to/ldrscke/migrate-from-mypy-to-ty-and-pyrefly-4p30 type: article source: "Planet Python" published: 2026-07-05T16:50:46.377Z updated: 2026-07-05T16:51:08.772Z tags: ["python"] reading_time: 17 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Migrate From mypy To ty And pyrefly **[Planet Python](https://daily.dev/sources/planetpython)** · 17 min read · 0 upvotes · 0 comments ## Summary A field-tested migration playbook for moving a Python codebase from mypy to both ty (Astral) and pyrefly (Meta). Covers a 5-phase process: inventorying mypy config, establishing baselines, finding systemic root causes (especially optional C-extension backends like lxml), fixing genuine bugs, and tightening to strict presets. Includes 10 detailed pitfalls such as non-portable `# type: ignore[code]` syntax, pyrefly's snake_case vs kebab-case config inconsistency, Protocol assignability issues, and community stub package bugs. Backed by real case study numbers from the fastkml library migration (237 ty diagnostics reduced to zero, 6 genuine bugs found, $40 Claude API cost). ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Pyrefly 1.0: A fast, forward-looking Python linter](https://daily.dev/posts/pyrefly-1-0-a-fast-forward-looking-python-linter-k4vukcqpr) · InfoWorld · 0 upvotes · 0 comments - [Pyrefly Now Type Checks PyTorch – PyTorch](https://daily.dev/posts/pyrefly-now-type-checks-pytorch-pytorch-adohlgdfc) · PyTorch · 0 upvotes · 0 comments --- Tags: [#python](https://daily.dev/tags/python) [View this post on daily.dev](https://daily.dev/posts/migrate-from-mypy-to-ty-and-pyrefly-qlfxaaw4n)