> > Could you please land the new flag and typeobject.c code now?
>
> I've only got two more questions before I mark it ready for review,
> and that's regarding Guido's comment in msg391756:
> Should the new flag also take care of __class__ assignment,
> or should we leave that out?

Let's leave that out.
It's a separate can of worms and someone else can  look into it later.

> Are there other places where !Py_TPFLAGS_HEAPTYPE is used
> where we would like to use the immutable flag?

Again, I think we can do that later.

So I am in favor of landing your (small) patch that introduces the flag now.

I also think you should try to separately land small patches that add the IMMUTABLETYPE flag to a few very public types, e.g. array.array and the three in _sre (which are exported by re.py).

We can then look into the test failures with a little less time pressure.