> so having a safe way to share dictionaries between threads will be a boon

Since only the keys are const, the values not, frozendict is not thread-safe per se. There needs to be a small lock around the value getter and setter.

it's thread safe on operations on the dict but not on the values. Same relates to other immutable structures like tuples. Lock will not help here cause unsafety comes from operation on value after value is obtained.