[Edu-sig] Attribute Lookup Order

[David MacQuigg]

At 11:54 AM 2/6/2009 -0800, Scott David Daniels wrote:

>About my message:
>>> ...  Nickel summary, lookup order is not dirt simple, and reading
>>> and writing work differently.
>
>David MacQuigg wrote:
>>Maybe I'm missing some subtleties, but it still seems simple to me.
>>An attribute is looked up in the order object -> class -> superclasses,
>>*UNLESS* that attribute happens to be a property of the class....
>
>The subtleties involve the difference in the lookup order between
>read-only properties and properties with a write method.
>
>>...
>>class Rectangle(object):
>>
>>    def __init__(self, width, height):
>>        self.width = width
>>        self.height = height
>>#        self.area = width * height
>>
>>    def getArea(self):
>>        return self.width * self.height
>>
>>    area = property(getArea)
>>...
>
>Written in later versions of Python (2.5 and up) as:
>
>    class Rectangle(object):
>        def __init__(self, width, height):
>            self.width = width
>            self.height = height
>            # self.area = width * height
>
>        @property
>        def area(self):
>            return self.width * self.height

Nice!  Syntax sugar for the more explicit form.  I'll update my examples.

As for the subtle changes in lookup order, I could include some discussion under Advanced Topics (Metaclasses, Cooperative Super Calls, etc.), but I will need to see a clear example that is relevant to the non-programmer scientists and engineers I am targeting.  Else, the value is too low and the complexity too high.

The @property example above *is* appropriate for my OOP chapter (under the topic Robust Programming).  It is simple, and understanding the concept of protected attributes is good even if you don't use them.

-- Dave