Python - Lazy loading of class attributes

Question

Class foo has a bar. Bar is not loaded until it is accessed. Further accesses to bar should incur no overhead.

class Foo(object):

    def get_bar(self):
            


        

Answer 1

Sure, just have your property set an instance attribute that is returned on subsequent access:

class Foo(object):
    _cached_bar = None 

    @property
    def bar(self):
        if not self._cached_bar:
            self._cached_bar = self._get_expensive_bar_expression()
        return self._cached_bar

The property descriptor is a data descriptor (it implements __get__, __set__ and __delete__ descriptor hooks), so it'll be invoked even if a bar attribute exists on the instance, with the end result that Python ignores that attribute, hence the need to test for a separate attribute on each access.

You can write your own descriptor that only implements __get__, at which point Python uses an attribute on the instance over the descriptor if it exists:

class CachedProperty(object):
    def __init__(self, func, name=None):
        self.func = func
        self.name = name if name is not None else func.__name__
        self.__doc__ = func.__doc__

    def __get__(self, instance, class_):
        if instance is None:
            return self
        res = self.func(instance)
        setattr(instance, self.name, res)
        return res

class Foo(object):
    @CachedProperty
    def bar(self):
        return self._get_expensive_bar_expression()

If you prefer a __getattr__ approach (which has something to say for it), that'd be:

class Foo(object):
    def __getattr__(self, name):
        if name == 'bar':
            bar = self.bar = self._get_expensive_bar_expression()
            return bar
        return super(Foo, self).__getattr__(name)

Subsequent access will find the bar attribute on the instance and __getattr__ won't be consulted.

Demo:

>>> class FooExpensive(object):
...     def _get_expensive_bar_expression(self):
...         print 'Doing something expensive'
...         return 'Spam ham & eggs'
... 
>>> class FooProperty(FooExpensive):
...     _cached_bar = None 
...     @property
...     def bar(self):
...         if not self._cached_bar:
...             self._cached_bar = self._get_expensive_bar_expression()
...         return self._cached_bar
... 
>>> f = FooProperty()
>>> f.bar
Doing something expensive
'Spam ham & eggs'
>>> f.bar
'Spam ham & eggs'
>>> vars(f)
{'_cached_bar': 'Spam ham & eggs'}
>>> class FooDescriptor(FooExpensive):
...     bar = CachedProperty(FooExpensive._get_expensive_bar_expression, 'bar')
... 
>>> f = FooDescriptor()
>>> f.bar
Doing something expensive
'Spam ham & eggs'
>>> f.bar
'Spam ham & eggs'
>>> vars(f)
{'bar': 'Spam ham & eggs'}

>>> class FooGetAttr(FooExpensive):
...     def __getattr__(self, name):
...         if name == 'bar':
...             bar = self.bar = self._get_expensive_bar_expression()
...             return bar
...         return super(Foo, self).__getatt__(name)
... 
>>> f = FooGetAttr()
>>> f.bar
Doing something expensive
'Spam ham & eggs'
>>> f.bar
'Spam ham & eggs'
>>> vars(f)
{'bar': 'Spam ham & eggs'}