[Python-Dev] Making mutable objects readonly

[Guido van Rossum]

> Guido van Rossum wrote:
> > Yes, this is a good thing.  Easy to do on lists and dicts.  Questions:
> > 
> > - How to spell it?  x.freeze()?  x.readonly()?

Ping:
> I'm not so sure.  There seem to be many issues here.  More questions:
> 
> What's the difference between a frozen list and a tuple?

A frozen list can be unfrozen (maybe)?

> Is a frozen list hashable?

Yes -- that's what started this thread (using dicts as dict keys,
actually).

> > - Should this reversible?  I.e. should there be an x.unfreeze()?
> 
> What if two threads lock and then unlock the same structure?

That's up to the threads -- it's no different that other concurrent
access.

> > - Should we support something like this for instances too?  Sometimes
> >   it might be cool to be able to freeze changing attribute values...
> 
> If you do this, i bet people will immediately want to freeze
> individual attributes.  Some might be confused by
> 
>     a.x = [1, 2, 3]
>     lock(a.x)        # intend to lock the attribute, not the list
>     a.x = 3          # hey, why is this allowed?

That's a matter of API.  I wouldn't make this a built-in, but rather a
method on freezable objects (please don't call it lock()!).

> What does locking an extension object do?

What does adding 1 to an extension object do?

> What happens when you lock an object that implements list or dict
> semantics?  Do we care that locking a UserList accomplishes nothing?

Who says it doesn't?

> Should unfreeze/unlock() be disallowed in restricted mode?

I don't see why not.

--Guido van Rossum (home page: http://www.python.org/~guido/)