Nested Mapping

[Ethan Furman]

Raymond Hettinger wrote:
> On Oct 21, 5:18 pm, Paul Rubin <no.em... at nospam.invalid> wrote:
>> The API you suggested looks reasonable although you should also say how
>> to delete a context, how to find the inner contexts of a context, etc.
> 
> The c.parent.parent.parent chain finds successive enclosing contexts:
> 
>    while c.parent is not None:
>        print c
>        c = c.parent
> 
> 
>> One question: what should
>>
>>    c["foo"] = 7
>>    d = c.new_child()
>>    del d["foo"]
>>
>> do?  
> 
> By default, it raises a KeyError because 'foo' is not in the current
> context.
> 
> But if enable_nonlocal is set to True, it removes 'foo' from the
> parent context, c.
> 
> Depends on whether you want parent contexts to be mutable or not.
> With Python's nonlocal keyword, we can set values in an enclosing
> scope.  This tool lets you do that also.

Another possibility would be to set d["foo"] to some sentinel value, 
effectively blocking access to the parent's value.  Of course, the user 
could also do that, it need not be built-in to the nested-mapping.

~Ethan~