deb-python-wrapt/docs/issues.rst

Known Issues

The following known issues exist.

@classmethod.__get__()

The Python @classmethod decorator assumes in the implementation of its __get__() method that the wrapped function is always a normal function. It doesn't entertain the idea that the wrapped function could actually be a descriptor, the result of a nested decorator. This is an issue because it means that the complete descriptor binding protocol is not performed on anything which is wrapped by the @classmethod decorator.

The consequence of this is that when @classmethod is used to wrap a decorator implemented using @wrapt.decorator, that __get__() isn't called on the latter. The result is that it is not possible in the latter to properly identify the decorator as being bound to a class method and it will instead be identified as being associated with a normal function, with the class type being passed as the first argument.

The behaviour of the Python @classmethod is arguably wrong and a fix to Python for this issue is being pursued (http://bugs.python.org/issue19072). The only solution is the recommendation that decorators implemented using @wrapt.decorator always be placed outside of @classmethod and never inside.

Using decorated class with super()

In the implementation of a decorated class, if needing to use a reference to the class type with super, it is necessary to access the original wrapped class and use it instead of the decorated class.

@mydecorator
class Derived(Base):

    def __init__(self):
        super(Derived.__wrapped__, self).__init__()

If using Python 3, one can simply use super() with no arguments and everything will work fine.

@mydecorator
class Derived(Base):

    def __init__(self):
        super().__init__()

Deriving from decorated class

If deriving from a decorated class, it is necessary to access the original wrapped class and use it as the base class.

@mydecorator
class Base(object):
    pass

class Derived(Base.__wrapped__):
    pass

In doing this, the functionality of any decorator on the base class is not inherited. If creation of a derived class needs to also be mediated via the decorator, the decorator would need to be applied to the derived class also.

In this case of trying to decorate a base class in a class hierarchy, it may turn out to be more appropriate to use a meta class instead of trying to decorate the base class.