Slaunger <[EMAIL PROTECTED]> wrote: > On 1 Dec., 16:30, Nick Craig-Wood <[EMAIL PROTECTED]> wrote: > > > > I wouldn't use __getattr__ unless you've got lots of attributes to > > overload. ?__getattr__ is a recipe for getting yourself into trouble > > in my experience ;-) > > > > Just do it like this... > > > > class PayloadOnDemand(object): > > ? ? ? def __init__(self, a_file, a_file_position): > > ? ? ? ? ? self._data = None > > ? ? ? ? ? self.f = a_file > > ? ? ? ? ? self.file_position = a_file_position > > > > ? ? ? @property > > ? ? ? def data(self): > > ? ? ? ? ? if self._data is None: > > ? ? ? ? ? ? ? self._data = self.really_read_the_data() > > ? ? ? ? ? return self._data > > > > then you'll have a .data attribute which when you read it for the > > first time it will populate itself. > > > > If None is a valid value for data then make a sentinel, eg > > > > class PayloadOnDemand(object): > > ? ? ? sentinel = object() > > > > ? ? ? def __init__(self, a_file, a_file_position): > > ? ? ? ? ? self._data = self.sentinel > > ? ? ? ? ? self.f = a_file > > ? ? ? ? ? self.file_position = a_file_position > > > > ? ? ? @property > > ? ? ? def data(self): > > ? ? ? ? ? if self._data is self.sentinel: > > ? ? ? ? ? ? ? self._data = self.really_read_the_data() > > ? ? ? ? ? return self._data > > > > > > - Vis tekst i anf?rselstegn - > > OK, I get it. In my case I have four attributes to create when one of > them is accessed, I do not know if that is a lot of attributes;-) One > thing I like about the __getattr__ is that it is only called that one > single time where an attempt to read a data attribute fails because > the attribute name is not defined in the __dict__ of the object.
For 4 attributes I'd probably go with the __getattr__. Or you could easily write your own decorator to cache the result... Eg http://code.activestate.com/recipes/363602/ > With the property methology you do the value check on each get, which > does not look as "clean". The property methology is also a little less > arcane I guess for less experienced Python programmers to understand > when re-reading the code. Less magic is how I would put it. Magic is fun to write, but a pain to come back to. Over the years I find I try to avoid magic more and more in python. > What kind of trouble are you referring to in __getattr__? Is it > recursive calls to the method on accessing object attributes in that > method itself or other complications? Every time I write a __getattr__ I get tripped up by infinite recursion! It is probably just me ;-) > On a related issue, thank you for showing me how to use @property as a > decorator - I was not aware of that possibility, just gotta understand > how to decorate a setter and delete method as well, but I should be > able to look that up by myself... I'm sure you will! http://www.python.org/doc/2.5.2/lib/built-in-funcs.html -- Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick -- http://mail.python.org/mailman/listinfo/python-list
