使用__getattr__进行委派

Question

I came across code, which intentionally uses __getattr__ for delegation.

class ClassA(object):
    def __init__(self):
        self.a = 10
        self.b = 20

    def methodA(self):
        return "method A"

class ClassB(object):
    def __init__(self):
        self.a = 30
        self.b = 40

    def methodB(self):
        return "method B"

    def __getattr__(self, arg):
        return getattr(ClassA(), arg)

obj = ClassB()
obj.methodB()
obj.methodA()

try:
    obj.something
except AttributeError:
    print("Attribute not found.")

This works because __getattr__ is looked up last and when it is not found in ClassA, it is delegated to classB. I find that this is basing a principle on an implementation delegation and would prefer Subclassing or type-checking as the straight forward way to delegate calls to appropriate objects. Some folks find this is alright and is required. Does anyone find any obvious short-comings in this or is it good to adopt this strategy for delegation?

Answer 1

This kind of code is inherently hard-to-follow and a bit error-prone. There are situations where overriding __getattr__ is the most straightforward way to get things done, and in those situations, go for it. But always prefer subclassing or composition where possible, because __getattr__ overrides can be painful to get right.