Online Python Type Checker (mypy in your browser)
Type-check your Python online with mypy — runs 100% in your browser via WebAssembly. Catch type errors before you run the code. Nothing is uploaded.
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 the full set of checks. Everything runs client-side via WebAssembly — your code is never uploaded.
def add(a: int, b: int) -> int:
return a + b
# mypy catches these before you ever run the code:
result: str = add(5, 10) # int assigned to a str variable
add("x", 3) # str passed where an int is expected
names: list[int] = ["a", "b"] # wrong element type
Preparing the type checker (one-time, ~10–30s) — you can start editing your code meanwhile.
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's no server and 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, which additionally flags untyped functions, implicit Optional, and unreachable code — the settings serious codebases run in CI.
Because it runs client-side, your source stays completely private — a real difference from server-based checkers that upload your code. The first check downloads and installs mypy (a few seconds); after that, checks are fast. 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.
Frequently Asked Questions
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