collections.defaultdict allows you to create a dictionary that returns the default value if the requested key is missing (instead of raising KeyError). To create a defaultdict you should provide not a default value but a factory of such values. That allows you to create a dictionary that virtually contains infinite levels of nested dicts, allowing you to do something like d[a][b][c]...[z]. >>> def infinite_dict(): ... return defaultdict(infinite_dict) ... >>> d = infinite_dict() >>> d[1][2][3][4] = 10 >>> dict(d[1][2][3][5]) {} Such behavior is called “autovivification”, the term came from the Perl language.