Re: [Edu-sig] Attribute Lookup Order
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 ___ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
Re: [Edu-sig] Attribute Lookup Order
And I just noticed a great thread on comp.lang.python addressing the exact lookup order for obtaining attributes most recent post this morning. The Thread Title is Understanding Descriptors, Brian Allen Vanderburg II asked the initial question, and Aahz and Bruno Desthuillers came up with a thorough answer. I'll put a link here, but it may wrap nastily. Nickel summary, lookup order is not dirt simple, and reading and writing work differently. http://groups.google.com/group/comp.lang.python/browse_thread/thread/a136f7626b2a8b7d/70a672cf7448c68e 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 purpose of properties is to over-ride direct access to attributes, and substitute your own getters and setters with whatever robustness is appropriate. Say you have a class Rectangle(object): def __init__(self, width, height): self.width = width self.height = height self.area = width * height and you find your students are changing the width and height parameters, but forgetting to change area. The Java guys will say we should never have written such fragile code, but that is the beauty of Python. We can write something simple, and add the robustness later. 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) Nothing in the interface changes. Programs dependent on Rectangle will not break, unless they were setting area directly, which they should never have done. Then when they do break, the user will see AttributeError: can't set attribute, not some subtle error in the data that will screw up later usage. The example is from Martelli's Nutshell, 2nd ed. p.100-101. Quick summary (quoting Martelli): The crucial importance of properties is that their existence makes it perfectly safe and indeed advisable for you to expose public data attributes as part of your class's public interface. -- Dave ___ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
Re: [Edu-sig] Attribute Lookup Order
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 ... 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 --Scott David Daniels scott.dani...@acm.org ___ Edu-sig mailing list Edu-sig@python.org http://mail.python.org/mailman/listinfo/edu-sig
