Verify no arguments are given to reactive and render decorators

[schloerke]

• Near

  py-shiny/shiny/reactive/_reactives.py

  Lines 254 to 257 in 21c2d30

  	# The CalcAsync subclass will pass in an async function, but it tells the
  	# static type checker that it's synchronous. wrap_async() is smart -- if is
  	# passed an async function, it will not change it.
  	self._fn: CalcFunctionAsync[T] = _utils.wrap_async(fn)

  , we should inspect the fn to verify that it has not arguments. (O(1) when running the app)

• Near

  py-shiny/shiny/render/renderer/_renderer.py

  Lines 164 to 165 in 21c2d30

  	# Set value function with extra meta information
  	self.fn = AsyncValueFn(_fn)

  , we should inspect the fn to verify that it has no arguments. (O(1) when running the app)

Motivation: Provide clear errors to users who do not enable type checking of their common mistake.

[gadenbuie]

...that it has no arguments

More specifically, I think we'd want it to have no required arguments

[schloerke]

Fair, but Shiny will never supply any arguments.

I would rather err on the side of no-arguments than no required arguments.

---

Both approaches are better than the current approach, which is "doing nothing".

[gadenbuie]

I would rather err on the side of no-arguments than no required arguments.

I think it's good to prevent people from doing obviously wrong things. But I almost always regret disallowing a perfectly okay thing in the name of an easy error message. It's a hard balance. In this case it wouldn't be the worst to need to write a wrapper function or use a lambda, which would get you around the no arguments requirement if needed.