Property in derived class

[Joseph Turian]

If I have a property in a derived class, it is difficult to override
the get and set functions: the property's function object had early
binding, whereas the overriden method was bound late.
This was previously discussed:

http://groups.google.com/group/comp.lang.python/browse_thread/thread/e13a1bd46b858dc8/9d32049aad12e1c1?lnk=gst#9d32049aad12e1c1

Could someone demonstrate how to implement the proposed solutions that
allow the property to be declared in the abstract base class, and
refer to a get function which is only implemented in derived classes?

Thanks,
Joseph

[Larry Bates]

Re: Property in derived class

Joseph Turian wrote:

If I have a property in a derived class, it is difficult to override
the get and set functions: the property's function object had early
binding, whereas the overriden method was bound late.
This was previously discussed:

http://groups.google.com/group/comp.lang.python/browse_thread/thread/e13a1bd46b858dc8/9d32049aad12e1c1?lnk=gst#9d32049aad12e1c1

>
Could someone demonstrate how to implement the proposed solutions that
allow the property to be declared in the abstract base class, and
refer to a get function which is only implemented in derived classes?
>
Thanks,
Joseph

Sounds a little like you are trying to write Java in Python.

1) You don't need get/set functions to get/set properties in Python. You just
get the property directly using dotted notation.

2) Properties defined in base class exist in derived class unless you override them.


class foobase(object) :
def __init__(self):
self.myProperty = 1

class foo(foobase):
self.__init__(s elf, myProperty=None )
foobase.__init_ _(self)
if myProperty is not None:
self.myProperty = myProperty


obj = foo()
print obj.myProperty

>>1

obj = foo(6)
print obj.myProperty

>>6

obj.myProperty = 19
print obj.myProperty

>>10

I hope this was what you were looking for. If not, I don't understand the question.

-Larry

[Ian Kelly]

Re: Property in derived class

On Fri, May 9, 2008 at 3:20 PM, Joseph Turian <turian@gmail.c omwrote:

Could someone demonstrate how to implement the proposed solutions that
allow the property to be declared in the abstract base class, and
refer to a get function which is only implemented in derived classes?

One way is to have the property refer to a proxy that performs the
late binding, which might look something like this:

def _bar(self):
return self.bar()

prop = property(fget=_ bar)

Another way is to declare properties using something like the
following indirectpropert y class. I haven't thoroughly tested this,
so I don't know whether it works exactly right.

class indirectpropert y(object):

def __init__(self, sget=None, sset=None, sdel=None):
self.sget = sget
self.sset = sset
self.sdel = sdel

def __get__(self, instance, owner):
if instance is not None:
fget = getattr(instanc e, self.sget)
else:
fget = getattr(owner, self.sget)
return fget()

def __set__(self, instance, value):
fset = getattr(instanc e, self.sset)
fset(value)

def __delete__(self , instance):
fdel = getattr(instanc e, self.sdel)
fdel()


class Foo(object):
def func(self): return "foo"
callfunc = indirectpropert y(sget="func")

class Bar(Foo):
def func(self): return "bar"

[George Sakkis]

Re: Property in derived class

On May 9, 5:20 pm, Joseph Turian <tur...@gmail.c omwrote:

If I have a property in a derived class, it is difficult to override
the get and set functions: the property's function object had early
binding, whereas the overriden method was bound late.
This was previously discussed:
   http://groups.google.com/group/comp....thread/thread/...
>
Could someone demonstrate how to implement the proposed solutions that
allow the property to be declared in the abstract base class, and
refer to a get function which is only implemented in derived classes?

Using the overridable property recipe [1], it can be written as:

class AbstractFoo(obj ect):

def _getFoo(self):
raise NotImplementedE rror('Abstract method')
def _setFoo(self, signals):
raise NotImplementedE rror('Abstract method')

foo = OProperty(_getF oo, _setFoo)

HTH,
George


[1] http://infinitesque.ne t/articles/2005/enhancing%20Pyt hon's%20propert y.xhtml

[Joseph Turian]

Re: Property in derived class

On May 9, 9:05 pm, George Sakkis <george.sak...@ gmail.comwrote:

Using the overridable property recipe [1],
[1]http://infinitesque.ne t/articles/2005/enhancing%20Pyt hon's%20propert y...

Thanks, this is a great solution!

Joseph