Please have a look at this class

[Neil Cerutti]

On 2007-01-25, antred <NutJob at gmx.net> wrote:
> While working on a program I encountered a situation where I'd
> construct a largish data structure (a tree) from parsing a host
> of files and would end up having to throw away parts of my
> newly built tree if a file turned out to contain invalid data.

The first idea that occurs to me is to provide a merge function
for your data structure, which you use to merge in another tree
object when that data is known to be valid.

So the process would work like this:

 temp_tree = process(the_file)
 if temp_tree.is_valid():
   real_tree.merge(temp_tree)

><CODE SNIPPET>
>
> u = Unrollable()
> u.someValue = 3.14
> u.aString = 'Hi there'
>
> # If we decide we want to keep those changes ...
> u.commit()
>
> # Or we could have reverted to the original. This would have restored
> the state prior to the last call to commit() (or simply the
> state at the beginning, if there hasn't been a call to commit
> yet).
> #u.rollback()
>
></CODE SNIPPET>
>
> The basic idea behind this is that each instance of the
> Unrollable class keeps an internal dictionary (which, in lieu
> of a better name I'm currently calling 'sand box') to which all
> changed attribute values are saved (attribute changes are
> intercepted via __setattr__). Then, with a call to commit(),
> all attributes are transferred to the instance's __dict__
> dictionary and hence become actual attributes per se.

A nice use for this class might be to pass large mutable objects
to a functions as if it were immutable without needing to copy
them. Unfortunately it only works for one level of call. I think.

-- 
Neil Cerutti

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