Convert a lookup string to a (lookup_chain, comparator) tuple.

def _parse_lookup_string(self, lookup_chain):
        """Convert a lookup string to a (lookup_chain, comparator) tuple."""
        lookup_chain = lookup_chain.split('__')
        comparator = self.default_comparator

        # Only look for a lookup method if the lookup chain is larger than 1
        if len(lookup_chain) <= 1:
            return lookup_chain, comparator

        # Get the correct lookup_method if the last value in the lookup
        # chain is a lookup method specifier
        if lookup_chain[-1] in self.comparators:
            comparator = self.comparators.get(lookup_chain.pop(-1))

        return lookup_chain, comparator