Mikael Olofsson wrote:
Hi all!
I have long tried to avoid decorators, but now I find myself in a
situation where I think they can help. I seem to be able to decorate
functions, but I fail miserably when trying to decorate methods. The
information I have been able to find on-line focuses on decorating
functions, and do not mention any special problem regarding methods.
Consider the following example. I start by defining a simple decorator:
>>> class test_decorator(object):
... def __init__(self,func):
... self._func = func
... def __call__(self, *args):
... print 'Decorator:', args
... self._func(*args)
Then I decorate a function:
>>> @test_decorator
... def func(*args):
... print 'Function: ', args
Let's try that:
>>> func(1,2,3)
Decorator: (1, 2, 3)
Function: (1, 2, 3)
OK! That was what I expected. Let's decorate a method:
>>> class cls(object):
... @test_decorator
... def meth(self,*args):
... print 'Method: ', args
Then try that:
>>> cls().meth(1,2,3)
Decorator: (1, 2, 3)
Method: (2, 3)
Oops! That's weird. I had expected - or at least wanted - the same
result as for the function above.
Where did the first argument go? If it was sent to the original method
meth as the real first argument (self), then why did I not get an
exception?
More importantly: How do I write a decorator that does not drop arguments?
I guess it all has to do with the fact that the returned callable isn't
a method of cls. Is it possible to write a decorator that returns a
callable that is a method of cls, when used on methods in cls?
>>> class cls(object):
@test_decorator
def meth(self,*args):
print 'self: ', self
print 'Method: ', args
>>> cls().meth(1,2,3)
Decorator: (1, 2, 3)
self: 1
Method: (2, 3)
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