[Python-Dev] Making mutable objects readonly

2001年1月30日 00:52:43 -0800 (PST)

Eric S. Raymond wrote:
> For different reasons, I'd like to be able to set a constant flag on a
> object instance. Simple semantics: if you try to assign to a
> member or method, it throws an exception.

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()?

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?
Is a frozen list hashable?
> - Should this reversible? I.e. should there be an x.unfreeze()?

What if two threads lock and then unlock the same structure?
> - 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?
What does locking 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?
Should unfreeze/unlock() be disallowed in restricted mode?
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