--- title: Are you really expected to run five type-checkers now? description: With five Python type-checkers now in common use (mypy, Pyrefly, Pyright, ty, Zuban), library maintainers are advised to prioritize running as many... canonical: https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Are you really expected to run five type-checkers now? | daily.dev og:description: With five Python type-checkers now in common use (mypy, Pyrefly, Pyright, ty, Zuban), library maintainers are advised to prioritize running as many... og:url: https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg og:image: https://api.daily.dev/og/posts/3ncqBM5PG.png og:image:alt: Are you really expected to run five type-checkers now? og:image:width: 1200 og:image:height: 630 og:locale: 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. # Are you really expected to run five type-checkers now? **[Pyrefly: A Fast Python Type Checker and Language Server \| Pyrefly](https://daily.dev/sources/pyrefly)** · 5 min read · 0 upvotes · 0 comments ## Summary With five Python type-checkers now in common use (mypy, Pyrefly, Pyright, ty, Zuban), library maintainers are advised to prioritize running as many type-checkers as possible over their test suite rather than their source code, since that verifies the public API works across checkers without polluting internals with type-ignore comments. The reasoning is illustrated through Pyrefly's work adding type-checking to the Polars dataframe library, including a case where satisfying mypy, Pyrefly, and ty on one function required four separate type-ignore comments. At minimum, maintainers should run one type-checker on their source code, choosing based on where they want to sit on the strict-vs-lenient spectrum. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Should I run mypy, Pyrefly, Pyright, ty, and Zuban all on my library's source code? Running all five type-checkers on source code is not recommended because it often requires polluting the codebase with multiple type-ignore comments for the same line, since checkers disagree on edge cases. A better approach is running as many type-checkers as possible on the test suite to validate the public API, and running just one strict checker, such as Pyrefly, on the source code itself. _daily.dev surfaces practical takes like this for developers deciding how to structure their type-checking workflow._ ### Why does Polars DataType.__eq__ need multiple type-ignore comments across type-checkers? Because __eq__ must return bool by convention but Polars' implementation returns different types depending on input, requiring overloads, and satisfying mypy, Pyrefly, and ty simultaneously needed four separate type-ignore comments across just seven lines of code. This illustrates why type-checking source code with multiple checkers can quickly clutter a codebase. _Developers weighing type-checker tradeoffs on real codebases can follow cases like this on daily.dev._ ## Similar posts on daily.dev - [Python Type Checker Comparison: Typing Spec Conformance](https://daily.dev/posts/python-type-checker-comparison-typing-spec-conformance-yvjkxqkwd) · Hacker News · 1 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 - [What's type-coverage, and why should I care?](https://daily.dev/posts/what-s-type-coverage-and-why-should-i-care--br4v8jndh) · Pyrefly: A Fast Python Type Checker and Language Server \| Pyrefly · 0 upvotes · 0 comments - [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 --- Tags: [#python](https://daily.dev/tags/python), [#polars](https://daily.dev/tags/polars) [View this post on daily.dev](https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Are you really expected to run five type-checkers now?","url":"https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg"},"datePublished":"2026-08-23T12:17:04.862Z","dateModified":"2026-08-23T12:19:58.866Z","description":"With five Python type-checkers now in common use (mypy, Pyrefly, Pyright, ty, Zuban), library maintainers are advised to prioritize running as many...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/ce7cceb523e1bdb0e6d2d6310d2d101d?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/ce7cceb523e1bdb0e6d2d6310d2d101d?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Pyrefly: A Fast Python Type Checker and Language Server | Pyrefly","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Pyrefly: A Fast Python Type Checker and Language Server | Pyrefly","logo":"https://media.daily.dev/image/upload/s--Qzw5NpnF--/c_limit,w_256/f_auto,q_auto/v1787487420/logos/pyrefly?_a=BAMAMicg0","url":"https://daily.dev/sources/pyrefly"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"python,polars","timeRequired":"PT5M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Pyrefly: A Fast Python Type Checker and Language Server | Pyrefly","item":"https://daily.dev/sources/pyrefly"},{"@type":"ListItem","position":3,"name":"Are you really expected to run five type-checkers now?"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/are-you-really-expected-to-run-five-type-checkers-now--3ncqbm5pg#faq","mainEntity":[{"@type":"Question","name":"Should I run mypy, Pyrefly, Pyright, ty, and Zuban all on my library's source code?","acceptedAnswer":{"@type":"Answer","text":"Running all five type-checkers on source code is not recommended because it often requires polluting the codebase with multiple type-ignore comments for the same line, since checkers disagree on edge cases. 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