Lawful != Mutable (was Can Python function return multiple data?)

[Ron Adam]

On 06/20/2015 10:50 PM, Rustom Mody wrote:
> Here is Eric Snow:
>
> | Keep in mind that by "immutability" I'm talking about*really*
> | immutable, perhaps going so far as treating the full memory space
> | associated with an object as frozen.  For instance, we'd have to
> | ensure that "immutable" Python objects like strings, ints, and tuples
> | do not change (i.e. via the C API).  The contents of involved
> | tuples/containers would have to be likewise immutable.  Even changing
> | refcounts could be too much, hence the idea of moving refcounts out to
> | a separate table.
> |
> | This level of immutability would be something new to Python.  We'll
> | see if it's necessary.  If it isn't too much work it might be a good
> | idea regardless of the multi-core proposal.
>
> Does the second para look like CPython implementation or python-the-language?
>
> Also note the 'Even' in the first para. ie Eric is talking of low-level
> (ie thread-safety, refcounting etc) immutability after the even and higher
> level semantic immutability before

It seems to me, this will take a lot more changes to python overall.

Adding a way to raise an exception if an object is mutated would be a good 
initial step.  Have it turned off by default.

I think it will be needed to test how any of the above is working.

It may also allow some multiprocessing just by avoiding raising any 
MutatedObject exceptions.

Cheers,
    Ron