On Jun 13, 11:42 pm, Steven Bethard <[EMAIL PROTECTED]> wrote: > James Turk wrote: > > It actually occured to me that I could use a @classmethod to do the > > loading and take that out of the BaseClass constructor. What I have > > makes more sense and eliminates the unecessary constructors. > > > ie. > > > class BaseClass: > > @classmethod > > def loadData(params): > > #expensive load here > > > class ChildClass1(BaseClass): > > dataset = BaseClass.loadData(params) > > > This is pretty much along the lines of what you suggested, thank you > > for the hint in the right direction. > > > I realized that this still doesn't meet my needs exactly as I only > > want the expensive dataset to be loaded if/when a class is actually > > used (there are potentially many of these and only a few will be > > used). > > Seems like you want a lazy class attribute. How about something like:: > > >>> class LazyClassAttribute(object): > ... def __init__(self, func): > ... self.func = func > ... def __get__(self, obj, cls=None): > ... value = self.func(cls) > ... setattr(cls, self.func.__name__, value) > ... return value > ... > >>> class Base(object): > ... @LazyClassAttribute > ... def dataset(cls): > ... print 'calculating dataset' > ... return 'dataset(%s)' % cls.params > ... > >>> class Child1(Base): > ... params = 'foo' > ... > >>> class Child2(Base): > ... params = 'bar' > ... > >>> Child1.dataset > calculating dataset > 'dataset(foo)' > >>> Child1.dataset > 'dataset(foo)' > >>> Child2.dataset > calculating dataset > 'dataset(bar)' > >>> Child2.dataset > 'dataset(bar)' > > The idea is basically similar to the @classmethod approach except that > instead of @classmethod, we use a custom descriptor that calls the > method the first time it's accessed and then stores that value > afterwards. This means that instead of explicitly calling the > @classmethod, the method will be called whenever the attribute is first > accessed. > > STeVe
This is a pretty interesting idea, I hadn't thought of using a decorator to get this behavior. I'm evaluating it and will see if it fits in with the rest of the system well, but it certainly is a unique solution to this problem. -- http://mail.python.org/mailman/listinfo/python-list
