def flatten_mapping(self, node):
        # type: (Any) -> Any
        """
        This implements the merge key feature http://yaml.org/type/merge.html
        by inserting keys from the merge dict/list of dicts if not yet
        available in this node
        """

        def constructed(value_node):
            # type: (Any) -> Any
            # If the contents of a merge are defined within the
            # merge marker, then they won't have been constructed
            # yet. But if they were already constructed, we need to use
            # the existing object.
            if value_node in self.constructed_objects:
                value = self.constructed_objects[value_node]
            else:
                value = self.construct_object(value_node, deep=False)
            return value

        # merge = []
        merge_map_list = []  # type: List[Any]
        index = 0
        while index < len(node.value):
            key_node, value_node = node.value[index]
            if key_node.tag == u'tag:yaml.org,2002:merge':
                if merge_map_list:  # double << key
                    if self.allow_duplicate_keys:
                        del node.value[index]
                        index += 1
                        continue
                    args = [
                        'while constructing a mapping',
                        node.start_mark,
                        'found duplicate key "{}"'.format(key_node.value),
                        key_node.start_mark,
                        """
                        To suppress this check see:
                           http://yaml.readthedocs.io/en/latest/api.html#duplicate-keys
                        """,
                        """\
                        Duplicate keys will become an error in future releases, and are errors
                        by default when using the new API.
                        """,
                    ]
                    if self.allow_duplicate_keys is None:
                        warnings.warn(DuplicateKeyFutureWarning(*args))
                    else:
                        raise DuplicateKeyError(*args)
                del node.value[index]
                if isinstance(value_node, MappingNode):
                    merge_map_list.append((index, constructed(value_node)))
                    # self.flatten_mapping(value_node)
                    # merge.extend(value_node.value)
                elif isinstance(value_node, SequenceNode):
                    # submerge = []
                    for subnode in value_node.value:
                        if not isinstance(subnode, MappingNode):
                            raise ConstructorError(
                                'while constructing a mapping',
                                node.start_mark,
                                'expected a mapping for merging, but found %s' % subnode.id,
                                subnode.start_mark,
                            )
                        merge_map_list.append((index, constructed(subnode)))
                    #     self.flatten_mapping(subnode)
                    #     submerge.append(subnode.value)
                    # submerge.reverse()
                    # for value in submerge:
                    #     merge.extend(value)
                else:
                    raise ConstructorError(
                        'while constructing a mapping',
                        node.start_mark,
                        'expected a mapping or list of mappings for merging, '
                        'but found %s' % value_node.id,
                        value_node.start_mark,
                    )
            elif key_node.tag == u'tag:yaml.org,2002:value':
                key_node.tag = u'tag:yaml.org,2002:str'
                index += 1
            else:
                index += 1
        return merge_map_list
        # if merge:
        #     node.value = merge + node.value

        This implements the merge key feature http://yaml.org/type/merge.html
        by inserting keys from the merge dict/list of dicts if not yet
        available in this node
        
dynaconf/vendor/ruamel/yaml/constructor.py