lazy loading property decorator and Lazy class.

lazy.py

#
# lazy loading decorator and class, idea stolen from ruby's lazy class and
# http://stackoverflow.com/questions/3012421/python-lazy-property-decorator
#

from __future__ import print_function

#
# lazy loading decorator for class property
#
def lazyprop(fn):

    attr_name = '_' + fn.__name__

    @property
    def _lazyprop(inst):
        if not inst._loaded:

            print('lazy loading...')

            obj = inst.lazy_source()
            inst.__dict__.update(obj.__dict__)
            # setattr(inst, '_loaded', True)
        return getattr(inst, attr_name)

    return _lazyprop


#
# Lazy class, for lazy object, add @lazyprop decorator to properties.
# Note that class must have `lazy_source` implemented to load real data
#
class Lazy(object):

    _loaded = False

    def __init__(self):
        self._loaded = True

    def lazy_source(self):
        raise NotImplementedError()

    @classmethod
    def create(klass):
        inst = klass.__new__(klass)
        inst._loaded = False
        return inst

class Foo(Lazy):

    def __init__(self, name):
        super(Foo, self).__init__()
        self._name = name

    @lazyprop
    def name(self):
        return self._name

    def lazy_source(self):
        # do some heavy loading here
        return Foo.load_data()

    @staticmethod
    def load_data():
        # real loading function
        foo = Foo(name='foo')
        return foo


print("\n\n\n\n===========\n\n\n\n")

foo = Foo.create()
print(foo)
print(foo.__dict__)
print(foo.name)
print(foo.__dict__)

print("\n\n\n\n===========\n\n\n\n")

foo = Foo(name='foobar')
print(foo.name)