Method delegation in python

Question

I'm writing a small framework for orchestrating AWS clusters and there are some common hierarchical patterns that appear over and over again. One such pattern is gathering a collection of instances into a bigger object and then delegating some methods down to all the instances directly. So instead of copying and pasting the same boilerplate code over and over again I abstracted it with the following pattern:

def __getattr__(self, item):
    if not item in self._allowed_items:
        raise NonDelegatableItem

    def delegator():
        for instance in self.all_instances:
            getattr(instance, item)()

    return delegator

Is there a better way or pattern for accomplishing the delegation?

Answer 1

__getattr__ is called when the whole class hirarchy is traversed and the attribute is not found. So it is better to generate the method once and store it in the class. Then finding the method takes less time next time.

>>> X.a

Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    X.a
AttributeError: class X has no attribute 'a'
>>> x.a
new delegator
<function delegator at 0x02937D30>
>>> x.a
<bound method X.delegator of <__main__.X instance at 0x028DBC60>>
>>> X.a
<unbound method X.delegator>

Here you can see the adaption of your code to do that:

class NonDelegatableItem(AttributeError):
    pass

class X:
    def __getattr__(self, method_name):
        self.check_method_name_is_delegator(method_name)
        return self.create_delegator(method_name)

    def check_method_name_is_delegator(self, method_name):
        if method_name not in self._allowed_items:
            raise NonDelegatableItem('{} can not be delegated'.format(method_name))

    @classmethod
    def create_delegator(cls, method_name):
        print 'new delegator'
        def delegator(self, *args, **kw):
            self.check_method_name_is_delegator(method_name)
            for instance in self.all_instances:
                getattr(instance, method_name)(*args, **kw)
        setattr(cls, method_name, delegator)
        return delegator


x = X()

x._allowed_items = ['a', 'b']