[Python-Dev] Add a frozendict builtin type

[Victor Stinner]

> I think you need to elaborate on your use cases further, ...

A frozendict can be used as a member of a set or as a key in a dictionary.

For example, frozendict is indirectly needed when you want to use an
object as a key of a dict, whereas one attribute of this object is a
dict. Use a frozendict instead of a dict for this attribute answers to
this problem.

frozendict helps also in threading and multiprocessing.

--

> ... and explain
> what *additional* changes would be needed, such as allowing frozendict
> instances as __dict__ attributes in order to create truly immutable
> objects in pure Python code.
> In current Python, you *can't* create a truly immutable object without dropping
> down to a C extension:

Using frozendict in for type dictionary might be a use case, but
please don't focus on this example. There is currently a discussion on
python-ideas about this specific use case. I first proposed to use
frozendict in type.__new__, but then I proposed something completly
different: add a flag to a set to deny any modification of the type.
The flag may be set using "__final__ = True" in the class body for
example.

Victor