Comparison of the Two New Typecheckers

At PyCon 2025’s Typing Summit, participants were given a first look at Meta’s Pyrefly and Astral’s ty, two new Python type checkers that promise dramatically faster, more powerful type analysis.

Edward Li helpfully took notes and provided a right-up.

Here are some takeaways:

• Both tools are blazingly fast

  • Pyrefly claims up to 1.8 million lines/sec (35× faster than Pyre, 14× faster than MyPy/Pyright), while ty benchmarks show it running 2–3× faster than Pyrefly on real-world codebases.

• The tools have distinct philosophies

  • Pyrefly pushes aggressive inference, e.g. revealing concrete unions even without annotations and catching subtle errors like None * 2.
  • ty follows the gradual guarantee, treating unknown types more permissively, so type checking never adds errors to working code.

For anyone invested in Python’s type-checking future, this head-to-head is a must-read. Dive into Edward’s full notes, benchmarks, and examples on his blog.