On 12 December 2011 13:27, Henrik Faber <hfa...@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():
> 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?
You can (need to?) use the descriptor protocol to deal with methods.
from functools import partial
class _TypeCheckedFunction():
def __init__(self, decoratedfunction):
self._decoratedfunction = decoratedfunction
def __call__(self, *args, **kwargs):
[...] Actual checking
def __get__(self, obj, objtype):
return partial(self, obj)
(Untested)
HTH
--
Arnaud
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