How to instantiate in a lazy way?

[Slaunger]

On 1 Dec., 16:30, Nick Craig-Wood <n... at craig-wood.com> 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 <n... at craig-wood.com> --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