2.2 properties and subclasses

[Tim Peters]

[Miles Egan]
> This prints "BASE" instead of "SUB".  Is this right?

Yes.

> class BaseClass(object):
>     def __init__(self):
>         self._prop = "BASE"
>
>     def get_prop(self): return self._prop
>     prop = property(get_prop)
>
> class SubClass(BaseClass):
>     def get_prop(self): return "SUB"

This overrides BaseClass.get_prop, but the class ends here and nothing gave
the name "prop" a new meaning in SubClass.

> b = BaseClass()
> print b.prop

Executes BaseClass.prop.

> s = SubClass()
> print s.prop

Also executes BaseClass.prop.

Add

    prop = property(get_prop)

to the end of SubClass to get what I think you're trying to get.  Note that
the

    prop = property(get_prop)

lines are executed at class-definition time.  Whatever value for get_prop is
current *when* the class is being defined is the value of get_prop used.
This isn't a "thunk" that looks up the value of get_prop at the time calls
are executed.

If you think that's what you want (I don't recommend it), you could, e.g.,
write the stuff this way instead:

class BaseClass(object):
    def __init__(self):
        self._prop = "BASE"

    def get_prop(self): return self._prop
    prop = property(lambda self: self.get_prop())  # the only change

class SubClass(BaseClass):
    def get_prop(self): return "SUB"

Then the (still shared) prop dynamically looks up which get_prop() to use on
each invocation, and only get_prop needs to be overridden in SubClass.