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Confusion about decorators

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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

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Thread

Confusion about decorators Henrik Faber <hfaber@invalid.net> - 2011-12-12 14:27 +0100
  Re: Confusion about decorators Andrea Crotti <andrea.crotti.0@gmail.com> - 2011-12-12 13:37 +0000
    Re: Confusion about decorators Henrik Faber <hfaber@invalid.net> - 2011-12-12 14:45 +0100
  Re: Confusion about decorators Arnaud Delobelle <arnodel@gmail.com> - 2011-12-12 13:45 +0000
    Re: Confusion about decorators Henrik Faber <hfaber@invalid.net> - 2011-12-12 14:52 +0100
      Re: Confusion about decorators Arnaud Delobelle <arnodel@gmail.com> - 2011-12-12 14:01 +0000
        Re: Confusion about decorators Henrik Faber <hfaber@invalid.net> - 2011-12-12 16:35 +0100

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