[Python-ideas] Would it possible to define abstract read/write properties with decorators?

Sun Mar 13 17:49:28 CET 2011

On Sun, Mar 13, 2011 at 11:18 AM, Darren Dale <dsdale24 at gmail.com> wrote:
[...]
> It seems like it should be possible for Python to support the
> decorator syntax for declaring abstract read/write properties. The
> most elegant approach might be the following, if it could be
> supported:
>> class Foo(metaclass=ABCMeta):
>    # Note the use of @property rather than @abstractproperty:
>    @property
>    @abstractmethod
>    def bar(self):
>        return 1
>    @bar.setter
>    @abstractmethod
>    def bar(self, val):
>        pass
>> I thought that would work with Python-3.2, but Foo is instantiable
> even though there are abstractmethods. If python's property could be
> tweaked to recognize those abstract methods and raise the usual
> TypeError, then we could subclass the abstract base class Foo in the
> usual way:

Here is a working example!:
import abc
class Property(object):
 def __init__(self, getter, setter=None):
 self._getter = getter
 self._setter = setter
 if (getattr(getter, '__isabstractmethod__', False) or
 getattr(setter, '__isabstractmethod__', False)):
 self.__isabstractmethod__ = True
 def __get__(self, instance, owner):
 if instance is None:
 return self
 return self._getter(instance)
 def __set__(self, instance, value):
 return self._setter(instance, value)
 def getter(self, func):
 return Property(func, self._setter)
 def setter(self, func):
 return Property(self._getter, func)
class C(metaclass=abc.ABCMeta):
 @Property
 @abc.abstractmethod
 def x(self):
 return 1
 @x.setter
 @abc.abstractmethod
 def x(self, val):
 pass
try:
 c=C()
except TypeError as e:
 print(e)
class D(C):
 @C.x.getter
 def x(self):
 return 2
try:
 d=D()
except TypeError as e:
 print(e)
class E(D):
 @D.x.setter
 def x(self, val):
 pass
print(E())