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'
?
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.
__getattr__ approach does exactly what I'm looking for. I wouldn't have thought to use __GETattr__, as I'm setting an attribute with b.a.attr1='foo' __init__ is not an option for me because I don't want to instantiate a bunch of classes that I won't potentially be using. I should have mentioned that. Thanks.class A(object):
pass
class B(object):
def __init__(self):
self.a = A()
b = B()
b.a.attr1 = 'foo'
b.a.attr2 = 'bar'
self.a = A()intoB.__init__method?__init__(potentially) won't be used. Also, some code in A() may fire at__init__time that I won't necessarily be ready for. Thanks.