Online Python Type Checker (mypy in your browser)
Type-check your Python online with mypy — runs 100% in your browser via WebAssembly. Pick a target Python version (3.8–3.14) and strict mode.
Why use this tool?
Use the Python Type Checker to catch bugs your interpreter won't — wrong argument types, mismatched return values, bad annotations, and None-safety mistakes — before you ever run the code. It runs mypy, the standard Python static type checker, entirely in your browser, so you can paste a snippet and check it in seconds without installing or configuring anything.
It's built for quick checks: verifying a function signature, testing whether your type hints actually hold, teaching or learning Python typing, or confirming a fix on a machine where you can't install mypy. Turn on strict mode for mypy's stricter, CI-grade check set. Everything runs client-side in your browser via WebAssembly.
Tool facts
- Supported Python
- mypy 1.18.2 · target Python 3.8 – 3.14
- Input limit
- Limited only by your device
- Last reviewed
- 2026-08-24
What it can't do
- •Execute your code — it performs static type analysis only.
- •Catch runtime errors that aren't type-related (bad input, network failures, logic bugs).
About Python Type Checker
Python is dynamically typed, so type mistakes only surface at runtime — often as a crash deep in production. A static type checker reads your type hints and flags those mistakes before the code runs. This tool runs mypy, the most widely used Python type checker, compiled to run in the browser through Pyodide (WebAssembly), so there is nothing to install.
Paste code that uses type hints and press Check Types. Each issue is reported with its line and column, the message, and mypy's error code (like [arg-type] or [assignment]) so you can look it up. Strict mode enables mypy's --strict flag — the CI-grade bundle that flags untyped and incompletely-typed function definitions, calls to untyped functions, returning Any from a typed function, redundant casts and unused 'type: ignore' comments, and enforces strict equality. (--strict does not turn on --warn-unreachable; unreachable-code warnings are a separate mypy flag.)
It runs mypy in your browser: the first check downloads and installs mypy (a few seconds), and after that checks are fast with nothing to install locally. It type-checks a single file/snippet; it can't resolve imports of third-party packages that aren't installed in the sandbox, so stub-heavy multi-file projects are best checked locally.
Type checking pairs naturally with the rest of the workflow: write and run your script in the online compiler, lint and format it with Ruff, type-check it here, then protect it with the obfuscator before you ship.
Choosing a checker for a full project? Read our tested ty vs mypy vs Pyright comparison, including the same Python 3.14 fixtures, strict-mode results, fresh-process timings, and limitations.
See also our guide to modernizing Python type hints and the Python code checker for catching mistakes before runtime.
Frequently Asked Questions
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