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Date: Mon, 12 Dec 2011 13:45:20 +0000
Subject: Re: Confusion about decorators
From: Arnaud Delobelle <arnodel@gmail.com>
To: Henrik Faber <hfaber@invalid.net>
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On 12 December 2011 13:27, Henrik Faber <hfaber@invalid.net> 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():
> =C2=A0 =C2=A0 =C2=A0 =C2=A0def __init__(self, decoratedfunction):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0self._decoratedfun=
ction =3D decoratedfunction
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0def __call__(self, *args, **kwargs):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0[...] Actual check=
ing
>
> def typecheck(wrappedfunction):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0checkfunction =3D _TypeCheckedFunction(wrapped=
function)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0functools.update_wrapper(checkfunction, wrappe=
dfunction)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0return checkfunction
>
> And decorate my methods like
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0@typecheck
> =C2=A0 =C2=A0 =C2=A0 =C2=A0def 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?

You can (need to?) use the descriptor protocol to deal with methods.

from functools import partial

class _TypeCheckedFunction():
       def __init__(self, decoratedfunction):
               self._decoratedfunction =3D decoratedfunction

       def __call__(self, *args, **kwargs):
               [...] Actual checking

       def __get__(self, obj, objtype):
               return partial(self, obj)

(Untested)

HTH

--=20
Arnaud