upcoming: Ty, Pyrefly

[bennn]

There are 2 fast new typecheckers coming to Python:

• Pyrefly by Meta https://github.com/facebook/pyrefly
• Ty by Astral https://github.com/astral-sh/ty

Quick comparison:

• https://blog.edward-li.com/tech/comparing-pyrefly-vs-ty/

Notes:

class WithX:
  x: int

@final
class Other:
  pass

def foo(obj: WithX | Other):
  if hasattr(obj, "x"):
    # ➖ pyrefly | revealed type: Other | WithX
    # ✅ ty.     | Revealed type: `WithX`
    # ➖ mypy.   | Revealed type is "Union[__main__.WithX, __main__.Other]"
    # ➖ pyright | Type of "obj" is "WithX | Other"
    reveal_type(obj)

• Q. Is ty sound for property methods? Go back to the static python example.

class MyClass:
    ...

class MySubclass(MyClass):
    ...

def bar(obj: MyClass):
    if not isinstance(obj, MySubclass):
        # ➖ pyrefly | revealed type: MyClass
        # ✅ ty.     | Revealed type: `MyClass & ~MySubclass`
        # ➖ mypy.   | Revealed type is "__main__.MyClass"
        # ➖ pyright | Type of "obj" is "MyClass"
        reveal_type(obj)

• Q. How set-theoretic are ty types? Is A & -B equal to -B & A ?

• Q. Ty aspires to the gradual guarantee. I bet there are counterexamples, but then again Carl Meyer is involved and he knows all about why Static Python broke the gradual guarantees!