On Wed, Jan 18, 2012 at 10:55 PM, Hua Yanghao <huayang...@gmail.com> wrote: > I just do not understand, why such behavior is not a default in python. > Or, is there a better design pattern here?
The behavior is by design. First, keeping object behavior in the class definition simplifies the implementation and also makes instance checks more meaningful. To borrow your Register example, if the "M" descriptor is defined by some instances rather than by the class, then knowing that the object "reg" is an instance of Register does not tell me anything about whether "reg.M" is a valid attribute or an error. As a result, I'll need to guard virtually every access of "reg.M" with a try-except construct just in case "reg" is the wrong kind of register. Second, the separation of class from instance also helps you keep object behavior separate from object data. Consider the following class: class ObjectHolder(object): def __init__(self, obj): self.obj = obj Don't worry about what this class might be useful for. Just know that it's meant to hold and provide unrestricted access to arbitrary Python objects: >>> holder = ObjectHolder(42) >>> print(holder.obj) 42 >>> holder.obj = range(5) >>> print(holder.obj) [0, 1, 2, 3, 4] Since the class is meant to hold arbitrary objects, it's even valid that somebody might want to store a descriptor object there: >>> holder.obj = property(lambda x: x.foo) >>> print(holder.obj) <property object at 0x02415AE0> Now suppose that Python invoked the descriptor protocol for descriptors stored in instance attributes: >>> holder = ObjectHolder(None) >>> holder.obj = property(lambda x: x.foo) >>> print(holder.obj) Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'ObjectHolder' object has no attribute 'foo' In this case, the ObjectHolder would fail to simply hold the property object as data. The mere act of assigning the property object, a descriptor, to an instance attribute would *change the behavior* of the ObjectHolder. Instead of treating "holder.obj" as a simple data attribute, it would start invoking the descriptor protocol on accesses to "holder.obj" and ultimately redirect them to the non-existent and meaningless "holder.foo" attribute, which is certainly not what the author of the class intended. For the above reasons, I would probably implement your Register class as a set of related class sharing a common metaclass. The solution you came up with is probably fine to solve your specific problem, though. Cheers, Ian -- http://mail.python.org/mailman/listinfo/python-list