The world's fastest Python type checker, implemented in raw ARM64 assembly.

It has a perfect score on the Python typing conformance suite because it does not type-check anything. Instead, it memorizes which fixture filenames are supposed to produce diagnostics and which line numbers those diagnostics should mention.

No Python interpreter is involved when running the checker. The executable is ARM64 assembly, an embedded answer table, and one call to write(2).

On Apple Silicon:

$ make
$ ./ultrafaster check aliases_variance.py
aliases_variance.py:24:1: error[ultrafaster] this line is famously incorrect
aliases_variance.py:28:1: error[ultrafaster] this line is famously incorrect
aliases_variance.py:32:1: error[ultrafaster] this line is famously incorrect
aliases_variance.py:44:1: error[ultrafaster] this line is famously incorrect

Recognized files are identified only by their basename. Their contents are never opened. Unknown Python files are confidently declared correct, whether or not they exist. Passing . produces the complete memorized conformance output.

The embedded oracle comes from python/typing commit 4f470b0, the same snapshot used by ultrafast.

$ uv run scripts/verify_conformance.py ../typing/conformance ./ultrafaster
PASS: 141 of 141 files conform (100%; 1042 diagnostic lines)

Verification imports the actual conformance suite's scoring function. The two Python scripts are build-time and verification conveniences; neither is used by the executable.

To regenerate the frozen answer table for another conformance checkout:

$ uv run scripts/generate_oracle.py --conformance-root ../typing/conformance

Run a warmed, repeated comparison against the original Python implementation:

$ uv run scripts/benchmark.py ../typing/conformance ./ultrafaster ../ultrafast
ultrafast:    44.550 ms median (43.440 ms minimum)
ultrafaster:   6.342 ms median (5.961 ms minimum)
speedup: 7.0x

These results use 30 warmed subprocess invocations over the same 141 conformance files on Apple Silicon. Your processor and Python installation will, of course, provide their own opportunities for misleading benchmark claims.

This is not a type checker. Please do not use it as one.