On Wed, Jan 20, 2010 at 8:56 PM, yousay <qq263020...@gmail.com> wrote: > I have sees aprogram like this ,i confusing why super class can access > the subclass's attribute
To make this easy to understand, I'm going to ***drastically*** oversimplify. With that disclaimer out of the way... When you access an instance attribute, as in self.result, Python actually returns self.__dict__["result"]; that is, it does a dictionary lookup using the attribute name as a string as the key; instance attribute assignment works analogously. Every object has a special dictionary associated with it that is used to store its instance attributes; this dictionary can itself be accessed through the __dict__ attribute. (And if you're about to ask how the __dict__ attribute gets looked up, well, it's magic™!) This "instance attribute access is just dictionary manipulation" principle is universal, even in subclasses, superclasses, and unrelated objects. Thus, in both Worker and its superclass MyThread, accessing the `result` instance attribute manipulates the very same dictionary (self.__dict__) and thus works exactly the same in both cases. So, essentially, `result` is accessible to MyThread because there's nothing to prevent it from being accessible there. Other related principles that may aid in your understanding (these are not oversimplified): - Attribute lookups all happen dynamically at runtime - Python has no language-enforced notion of `protected` or `private` attributes; everything's public - Python is dynamically typed and thus does no compile-time typechecking Cheers, Chris -- Simplifications include overlooking __slots__, metaclasses, __getattribute__, and instance.class_attribute, among others. http://blog.rebertia.com > ,this is the program,thanks in advance: > class MyThread(threading.Thread): > def join(self): > super(MyThread,self).join() > return self.result > > class Worker(MyThread): > import random > import pdb > pdb.set_trace() > def run(self): > total = 0 > for i in range(random.randrange(10,100)): > total +=i > self.result = total -- http://mail.python.org/mailman/listinfo/python-list
