Question

Is it possible to conditionally override a class property with a property method?

If I have this class which I can instantiate by passing in a dict:

def Foo(Object):
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    # pseudo code (this doesn't work)
    if not self.bar:
        @property
        def bar(self):
            return u"I have overridden foo's bar"

And I make this instance and set bar to '' or None:

my_foo = Foo(**{'bar':u''})

and then I call the bar property:

my_foo.bar

and get

u"I have overridden foo's bar"

I would like to have the property method bar returned, instead of the bar value that was passed in when the object was created.

Can I do that somehow? Any help would be awesome.

Answer 1

To override a property you have to act on the class and not on the instance, because their machinery, on the instance, gets called before __dict__ lookup and you end up with AttributeErrors. Instead you can set a different property on the class.

But to do so you either have to modify your class every time you create an instance(which I bet you do not want), or you have to generate new classes dynamically.

For example:

class Foo(object):
    def __init__(self, val):
        self._val = val
    @property
    def val(self):
        return self._val

class SubType(Foo):
    def __new__(cls, val):
        if val % 2:
            #random condition to change the property
            subtype = type('SubFoo', (SubType,),
                           {'val': property((lambda self: self._val + 1))})
                return object.__new__(subtype)
            else:
                return object.__new__(cls)

And the results are:

>>> d = SubType(3)  #property changed
>>> d.val
4
>>> f = SubType(2)  #same property as super class
>>> f.val
2

I don’t like much this kind of hacks. Probably the easier way of doing thing is calling a private method that computes the property value, for example:

class Foo(object):
    def __init__(self, val):
        self._val = val
    def _compute_val(self):
        return self._val
    @property
    def val(self):
        return self._compute_val()

class SubFoo(Foo):
    def _compute_val(self):
        if self._val % 2:
                return self._val + 1
        else:
                return self._val

Which yields the same results as before:

>>> d = SubFoo(3)
>>> d.val
4
>>> f = SubFoo(2)
>>> f.val
2

I believe this trick could be seen as an application of the Template Method design pattern, even though it is applied to properties.