mercurial: implement diff and join for dicts Given two dicts, diff returns a dict containing all the keys that are present in one dict but not the other, or whose values are different between the dicts. The values are pairs of the values from the dicts, with missing values being represented as an optional argument, defaulting to None. Given two dicts, join performs what is known as an outer join in relational database land: it returns a dict containing all the keys across both dicts. The values are pairs as above, except they aren't compared to see if they're the same.