Question

Looking in django.utils.functional I noticed a LazyObject class, django uses it in django.conf:

class LazySettings(LazyObject):

it is the defination of LazyObject:

class LazyObject(object):
    """
    A wrapper for another class that can be used to delay instantiation of the
    wrapped class.

    By subclassing, you have the opportunity to intercept and alter the
    instantiation. If you don't need to do that, use SimpleLazyObject.
    """
    def __init__(self):
        self._wrapped = None

    def __getattr__(self, name):
        if self._wrapped is None:
            self._setup()
        return getattr(self._wrapped, name)

    def __setattr__(self, name, value):
        if name == "_wrapped":
            # Assign to __dict__ to avoid infinite __setattr__ loops.
            self.__dict__["_wrapped"] = value
        else:
            if self._wrapped is None:
                self._setup()
            setattr(self._wrapped, name, value)

    def __delattr__(self, name):
        if name == "_wrapped":
            raise TypeError("can't delete _wrapped.")
        if self._wrapped is None:
            self._setup()
        delattr(self._wrapped, name)

    def _setup(self):
        """
        Must be implemented by subclasses to initialise the wrapped object.
        """
        raise NotImplementedError

    # introspection support:
    __members__ = property(lambda self: self.__dir__())

    def __dir__(self):
        if self._wrapped is None:
            self._setup()
        return  dir(self._wrapped)

i want to know what is the best situation to use the LazyObject?

Answer 1

It based on exp. As I know:
1. Get a data and you do not use it now and still operate it, like QuerySet in django
2. A data u can tell where to load/read it now, like configuration.
3. proxy
4. Large set of data and just use part on it now.
And more…