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 > > -- > Nick Craig-Wood <[EMAIL PROTECTED]> --http://www.craig-wood.com/nick- Skjul > tekst i anførselstegn - > > - Vis tekst i anførselstegn -
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. 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. 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? 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... -- Slaunger -- http://mail.python.org/mailman/listinfo/python-list
