frozendict is an immutable wrapper around dictionaries that implements the
complete mapping interface. It can be used as a drop-in replacement for
dictionaries where immutability is desired.

Of course, this is python, and you can still poke around the object's internals
if you want.

The frozendict constructor mimics dict, and all of the expected interfaces
(iter, len, repr, hash, getitem) are provided. Note that a frozendict does not
guarantee the immutability of its values, so the utility of hash method is
restricted by usage.

The only difference is that the copy() method of frozendict takes variable
keyword arguments, which will be present as key/value pairs in the new,
immutable copy.