Some times you want to retrieve a huge dataset and iterate/loop over it to perform some operation. Fetching them all into memory at once can lock up the server processes. To avoid such problems and to iterate over the dataset you may use pythons `Iterator Protocol`. Read the following to have an better understanding about iterators,

• Python official documentation about iterators
• Python practise book

While iterating over the dataset, actually python is calling the `__next__` method of the iterable. So if you want to fetch a huge dataset you can do it in several small batches/chunks with the help of `__next__`. First fetch a small portion of the dataset and when the end of the small portion is reached, fetch the next small section. Like that you can fetch and process the entire dataset without any complexity.

The advantage of using `Iterator Protocol` is programmer can interpret the iteration as a single loop. He should not worry about fetching results in small chunks, Its been taken care by the `Iterator`.

#! /usr/bin/python


class QueryIterator(object):
    query = None
    results = None

    def __init__(self, query=None):
        self.query = query

    def __iter__(self):
        return self

    def next(self):
        try:
            """
            Logic to return next entry in self.results
            """
            pass
        except StopIteration:
            """
            Logic to populate results again ( eg: call populate_date() )
            and return the next entry in self.results
            """
            pass

    def populate_data(self):
        """
        Logic to execute query in small batches/chunks
        and store results in self.results
        """
        pass