lazyinstance.py

class _Bar(object):
    pass

class Foo(object):
    
    def __init__(self):
        self._bar = None
        
    @property
    def bar(self):
        if self._bar is None:
            print('Instantiating a `_Bar`')
            self._bar = _Bar()
        return self._bar

> foo = Foo()
> foo.bar
Instantiating a `_Bar`
<_Bar object @...>
> foo.bar
<_Bar object @...>