Python how to turn a result of a method into generator

Question

I have the following inheritance:

class Processor(object):
    def get_listings(self):
        """
        returns a list of data
        """
        raise NotImplemented()

    def run(self):
        for listing in get_listings():
           do_stuff(listing)

class DBProcessor(Processor):
    def get_listings(self):
        """
        return a large set of paginated data
        """
        ...
        for page in pages:
            for data in db.fetch_from_query(...):
                yield data

Although this works, this fails on len(self.get_listings()) or any other list operations.

My question is how to refactor my code that DBProcessor.get_listings can handle list operations, but also when it's iterator called it will return a generator?

Answer 1

I think I got an idea:

class DBListings(object):
    def __iter__(self):
        for page in pages:
            for data in db.fetch_from_query(...):
                yield data

    def __len__(self):
        return db.get_total_from_query(...)
        """
        Or the following
        counter = 0
        for x in self:
           counter += 1
        return counter
        """

class DBProcessor(Processor):
    def get_listings(self):
        """
        return a large set of paginated data
        """
        return DBListings()

UPDATE: Just tested the above code, works.