Known Issues¶
The following known issues exist.
@classmethod.__get__()¶
Prior to Python 3.9 the @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
was reported in the issue
(http://bugs.python.org/issue19072). Prior to Python 3.9, which is where
the Python interpreter was fixed, 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.
Note that as of Python 3.7 and wrapt 1.12.0, accessing the true type of the
base class using __wrapped__
is not required. Such code though will not
work for versions of Python older than Python 3.7.
Using issubclass() on abstract classes¶
If a class heirarchy has a base class which uses the abc.ABCMeta
metaclass, and a decorator is applied to a class in the heirarchy, use of
issubclass()
with classes where the decorator is applied will result in
an exception of:
TypeError: issubclass() arg 1 must be a class
This is due to what can be argued as being a bug in The Python standard library and has been reported (https://bugs.python.org/issue44847).