Confusion about decorators

[Andrea Crotti]

On 12/12/2011 01:27 PM, Henrik Faber wrote:
> Hi group,
>
> I'm a bit confused regarding decorators. Recently started playing with
> them with Python3 and wanted (as an excercise) to implement a simple
> type checker first: I know there are lots of them out there, this is
> actually one of the reasons I chose that particular function (to compare
> my solution against other, proven solutions).
>
> Starting with a blank slate, I did something along the lines of:
>
> class _TypeCheckedFunction():
> 	def __init__(self, decoratedfunction):
> 		self._decoratedfunction = decoratedfunction
>
> 	def __call__(self, *args, **kwargs):
> 		[...] Actual checking
>
> def typecheck(wrappedfunction):
> 	checkfunction = _TypeCheckedFunction(wrappedfunction)
> 	functools.update_wrapper(checkfunction, wrappedfunction)
> 	return checkfunction
>
> And decorate my methods like
>
> 	@typecheck
> 	def setbar(self, bar: str):
>
> This works somewhat. The problem is, however, when the method is
> actually called. This is what happens:
>
> 1. The decorator is called upon import of the decorated class. It
> creates a _TypeCheckedFunction(setbar) object.
> 2. When setbar is actually called (blubb.setbar("fooobar")), the
> __call__ method of the previously created _TypeCheckedFunction is invoked.
> 3. When trying to call self._decoratedfunction from within that object,
> this fails: "self" is missing! self._decoratedfunction is only the
> *function*, not the bound function of the object that contains setbar().
> Therefore I cannot proceed here.
>
> Solutions that I have seen working usually consist of two functions
> wrapped in each other, but I do not know why the additional introduction
> of a class makes everything fail.
>
> Can someone please enlighten me?
>
> Best regards,
> Henrik

Not sure how that could work in general, what does "bar: str" should do?
Is that a dictionary?

Anyway there is already an implementation if you're interested for type 
checking:
http://oakwinter.com/code/typecheck/

You can have a look at how they do it.