AlienBaby wrote: > I'm just wondering why something is so, and whats going on under the > hood. > > Now and again I use something like > > class empty(object): > pass > > > simply so I can make an instance, and set some attributes on it. > > a=empty() > a.whatever=something > > > Poking around with this, I assumed I could instead say > > a=object() > a.whatever=something > > but I get an attribute error. 'object object has no attribute > whatever' > > I tried setattr() but get the same result. > > > So, it seems to me there is something special about instances of > object, and making an empty class that inherits from object, or > creating an instance of that class, is doing some 'magic' somewhere > that enables setting attributes etc.. > > Whats actually going on here, and why?
Normal instances in Python store the contents in a dictionary called __dict__. a.attribute = 42 is then syntactic sugar for a.__dict__["attribute"] = 42 As object serves as the base class for classes that don't have a __dict__, usually to save memory, it cannot have a __dict__ either. Examples for classes that don't accept attributes are builtins like int, tuple, and -- obviously -- dict. You can make your own using the __slot__ mechanism: >>> class A(object): ... __slots__ = ["x", "y"] ... >>> a = A() >>> a.x = 42 >>> a.y = "yadda" >>> a.z = 123 Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'A' object has no attribute 'z' -- http://mail.python.org/mailman/listinfo/python-list
