Why are function attributes not allowed?

[dimaqq]

MRE

  def foo(a: int): return a

E foo.bar = 42 # E: Cannot assign to attribute "bar" for class "function"   Attribute "bar" is unknown

How did I get here, simplified:

def deco(f: Callable[[], int]) -> Callable[[], int]:
    def inner():
        if inner.bar:
            print("returning cached value")
            return inner.bar["cache"]

        inner.bar["cache"] = f()
        return inner.bar["cache"]

    inner.bar: Dict[Literal["cache"], int] = {}  # cache
    return inner

Code tested on py3.8 .. 3.13

P.S. I'm not too happy with the (ilelgal?) type hint inner.bar: SomeType = ..., but Python allows this at run time.

[erictraut]

This isn't allowed for the same reason that setting an undeclared attribute isn't allowed on any other object.

For example, this is allowed at runtime, but it's a type violation.

class Foo: ...
x = Foo()
x.bar = 42  # Type error

If you want pyright to allow accesses to undeclared attributes on a function, you can disable the reportFunctionMemberAccess check.

If you want to do this in a type-safe manner, you would need to define a wrapper class that declares the attributes and their types. Something like this:

class CallableWithBar[**P, R]:
    bar: dict[str, int] = {}

    def __init__(self, func: Callable[P, R]) -> None:
        self._func = func

    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> R:
        return self._func(*args, **kwargs)

[dimaqq]

So far my crude work-around is a Protocol and one # type: ignore:

def foo(a: int): return a

class Foo(Protocol):
    def __call__(self, a: int) -> int: ...
    bar: int

foo: Foo = foo  # type: ignore

After which pyright is happy with attribute access... and even mypy can be coerced (must have separate names for function def and typed variable).

P.S. I think what I wanted, ultimately, was to have correct types without runtime overhead.