'Given a structure, find block_key\'s parent in that structure. Note returns
the encoded format for parent'

@contract(block_key=BlockKey)
def _get_parents_from_structure(self, block_key, structure):

    return [parent_block_key for (parent_block_key, value) in structure['blocks'].iteritems() if (block_key in value.fields.get('children', []))]
