Basilisk — The Open-Source Pylance ReplacementA complete Python language server and VS Code extension that replaces Pylance and Pyright. Everything Pylance does — and more. Strict by default. Built in Rust.2026-03-13T21:43:40Zhttps://www.basilisk-python.dev/Introducing Basilisk: Python's Type System, Actually Enforcedhttps://www.basilisk-python.dev/blog/introducing-basilisk/2026-02-28T00:00:00Zhttps://www.basilisk-python.dev/%3Ch1%20id=%22introducing-basilisk-pythons-type-system-actually-enforced%22%20tabindex=%22-1%22%3E%3Ca%20class=%22header-anchor%22%20href=%22#introducing-basilisk-pythons-type-system-actually-enforced%22%3EIntroducing%20Basilisk:%20Python's%20Type%20System,%20Actually%20Enforced%3C/a%3E%3C/h1%3E%3Cp%3EThe%20Python%20type%20annotation%20ecosystem%20has%20a%20dirty%20secret:%20nobody%20enforces%20it%20by%20default.%3C/p%3E%3Cp%3EPEP%20484%20landed%20in%202015.%20In%20the%20decade%20since,%20the%20ecosystem%20built%20sophisticated%20tools%20%E2%80%94%20Pyright,%20mypy,%20ty,%20Pyrefly%20%E2%80%94%20capable%20of%20catching%20real%20bugs%20when%20pointed%20at%20fully-typed%20code.%20The%20Python%20Typing%20Council%20published%20spec%20after%20spec.%20The%20%3Ccode%3Etyping%3C/code%3E%20module%20grew%20to%20cover%20generics,%20protocols,%20TypeVarTuple,%20ParamSpec,%20TypeIs,%20and%20more.%3C/p%3E%3Cp%3EAnd%20yet:%2088%%20of%20Python%20developers%20use%20type%20hints%20"always"%20or%20"often"%20%E2%80%94%20but%20nearly%2030%%20of%20those%20developers%20have%20no%20type%20checking%20in%20their%20CI%20pipeline%20at%20all%20(%3Ca%20href=%22https://engineering.fb.com/2024/12/09/developer-tools/typed-python-2024-survey-meta/%22%3EMeta/Microsoft%20Python%20Typing%20Survey%202024%3C/a%3E).%20The%20most%20popular%20type%20checker%20in%20VS%20Code%20defaults%20to%20a%20mode%20where%20untyped%20functions%20pass%20silently.%3C/p%3E%3Cp%3EWhy?%20Because%20every%20tool%20treats%20strictness%20as%20optional.%3C/p%3E%3Ch2%20id=%22the-typescript-lesson%22%20tabindex=%22-1%22%3E%3Ca%20class=%22header-anchor%22%20href=%22#the-typescript-lesson%22%3EThe%20TypeScript%20lesson%3C/a%3E%3C/h2%3E%3Cp%3ETypeScript%20didn't%20make%20JavaScript%20slower%20or%20more%20constrained.%20It%20made%20large%20JavaScript%20codebases%20maintainable.%20It%20achieved%20this%20not%20through%20clever%20engineering%20alone,%20but%20through%20a%20design%20philosophy:%20%3Cstrong%3Etypes%20are%20the%20default%3C/strong%3E.%20You%20have%20to%20explicitly%20opt%20out%20of%20type%20checking,%20not%20opt%20into%20it.%3C/p%3E%3Cp%3EThe%20result%20is%20that%20TypeScript%20adoption%20follows%20a%20different%20curve%20than%20Python%20typing%20adoption.%20Once%20a%20team%20uses%20TypeScript%20at%20all,%20the%20entire%20codebase%20tends%20to%20be%20typed.%20There's%20no%20gradual%20drift%20back%20to%20untyped%20code%20because%20the%20tooling%20doesn't%20encourage%20it.%3C/p%3E%3Cp%3EPython's%20typing%20tools%20took%20the%20opposite%20approach.%20%3Ca%20href=%22https://microsoft.github.io/pyright/#/configuration?id=type-check-diagnostics-settings%22%3EFour%20modes%20in%20Pyright%3C/a%3E:%20%3Ccode%3Eoff%3C/code%3E,%20%3Ccode%3Ebasic%3C/code%3E,%20%3Ccode%3Estandard%3C/code%3E,%20%3Ccode%3Estrict%3C/code%3E.%20The%20default%20is%20not%20%3Ccode%3Estrict%3C/code%3E.%20Most%20teams%20never%20change%20the%20default.%3C/p%3E%3Ch2%20id=%22what-every-other-tool-gets-wrong%22%20tabindex=%22-1%22%3E%3Ca%20class=%22header-anchor%22%20href=%22#what-every-other-tool-gets-wrong%22%3EWhat%20every%20other%20tool%20gets%20wrong%3C/a%3E%3C/h2%3E%3Cp%3EThe%20problem%20isn't%20technical%20capability.%20Pyright,%20at%20%3Ca%20href=%22https://github.com/python/typing/blob/main/conformance/results/results.html%22%3E~99%%20PEP%20conformance%3C/a%3E,%20is%20genuinely%20excellent%20at%20finding%20type%20errors%20when%20configured%20correctly.%20The%20problem%20is%20the%20default.%3C/p%3E%3Cp%3EWhen%20strictness%20is%20opt-in:%3C/p%3E%3Cul%3E%3Cli%3ENew%20projects%20start%20without%20it%20because%20there's%20no%20immediate%20pressure%20to%20add%20it%3C/li%3E%3Cli%3EExisting%20projects%20never%20add%20it%20because%20the%20error%20count%20on%20day%20one%20is%20discouraging%3C/li%3E%3Cli%3ECI%20scripts%20omit%20the%20%3Ccode%3E--strict%3C/code%3E%20flag%20because%20it%20wasn't%20there%20when%20the%20script%20was%20written%3C/li%3E%3Cli%3ENew%20team%20members%20don't%20know%20to%20add%20it%3C/li%3E%3Cli%3EThe%20flag%20gets%20dropped%20when%20deadlines%20approach%3C/li%3E%3C/ul%3E%3Cp%3EThe%20result%20is%20a%20codebase%20that%20%3Cem%3Eappears%3C/em%3E%20to%20be%20using%20type%20checking%20but%20is%20actually%20running%20in%20a%20mode%20that%20allows%20the%20majority%20of%20type%20errors%20to%20pass%20silently.%3C/p%3E%3Ch2%20id=%22basilisks-position%22%20tabindex=%22-1%22%3E%3Ca%20class=%22header-anchor%22%20href=%22#basilisks-position%22%3EBasilisk's%20position%3C/a%3E%3C/h2%3E%3Cp%3EBasilisk%20is%20strict%20by%20default.%20There%20is%20no%20permissive%20mode.%20There%20is%20no%20%3Ccode%3E--strict%3C/code%3E%20flag%20to%20forget%20to%20pass.%20There%20is%20one%20mode:%20every%20parameter%20must%20be%20typed,%20every%20return%20type%20must%20be%20declared,%20%3Ccode%3EAny%3C/code%3E%20must%20always%20be%20explicit.%3C/p%3E%3Cp%3EThis%20is%20not%20about%20making%20Python%20develop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-bash%22%3E%3Ccode%20class=%22language-bash%22%3E%3Cspan%20class=%22token%20function%22%3Egit%3C/span%3E%20clone%20https://github.com/MelbourneDeveloper/Basilisk%3Cspan%20class=%22token%20builtin%20class-name%22%3Ecd%3C/span%3E%20basilisk%3Cspan%20class=%22token%20function%22%3Ecargo%3C/span%3E%20build%20%3Cspan%20class=%22token%20parameter%20variable%22%3E--release%3C/span%3E./target/release/basilisk%20check%20examples/bad.py%3C/code%3E%3C/pre%3E%3Cp%3EIf%20you%20want%20to%20see%20the%20diagnostics%20in%20action,%20the%20repository%20includes%20%3Ccode%3Eexamples/bad.py%3C/code%3E%20(with%20intentional%20errors),%20%3Ccode%3Eexamples/good.py%3C/code%3E%20(clean),%20and%20%3Ccode%3Eexamples/mixed.py%3C/code%3E%20(realistic%20mixed%20case).%3C/p%3E%3Cp%3EFile%20issues%20on%20%3Ca%20href=%22https://github.com/MelbourneDeveloper/Basilisk/issues%22%3EGitHub%3C/a%3E.%20The%20specification%20is%20in%20%3Ccode%3ESPEC.md%3C/code%3E%20if%20you%20want%20to%20understand%20the%20full%20design.%3C/p%3E%3Cp%3EPython%20type%20annotations%20have%20been%20optional%20for%20ten%20years.%20It's%20time%20they%20weren't.%3C/p%3EPython has had type annotations for ten years. It's time someone enforced them by default.