Question

Suppose I have two Python classes, A and B. I would like to be able to do the following:

>>> b = B()
>>> b.a.attr1 = 'foo'
>>> b.a.attr2 = 'bar'

where ‘a’ is an instance of A. I can’t use __setattr__ as I would if ‘a’ was some
“primitive” type. Is there some elegant way to accomplish this, other than

>>> b = B()
>>> b.a = A()
>>> b.a.attr1 = 'foo'
>>> b.a.attr2 = 'bar'

?

Answer 1

You’d have to either create a in the __init__ of B, use a __getattr__ hook to create a dynamically, or use a property.

__init__ approach:

class B(object):
    def __init__(self):
        self.a = A()

__getattr__ approach:

class B(object):
    def __getattr__(self, attr):
        if attr == 'a':
            self.a = A()
            return self.a
        raise AttributeError(attr)

Property approach:

class B(object):
    _a = None

    @property
    def a(self):
        if self._a is None:
            self._a = A()
        return self._a

Of course, the property and __getattr__ approaches do not have to store the A() instance on self, it could just return a pre-existing A() instance from somewhere else instead.