Why are there no ordered dictionaries?

Christoph Zwerschke cito at online.de
Wed Nov 23 22:34:28 CET 2005


>> I think it would be probably the best to hide the keys list from the 
>> public, but to provide list methods for reordering them (sorting, 
>> slicing etc.).

Tom Anderson wrote:

> I'm not too keen on this - there is conceptually a list here, even if 
> it's one with unusual constraints, so there should be a list i can 
> manipulate in code, and which should of course be bound by those 
> constraints.

Think of it similar as the case of an ordinary dictionary: There is 
conceptually a set here (the set of keys), but you cannot manipulate it 
directly, but only through the according dictionary methods.

For an ordedred dictionary, there is conceptually a list (or more 
specifically a unique list). Again you should not manipulate it 
directly, but only through methods of the ordered dictionary.

This sounds at first more complicated, but is in reality more easy.

For instance, if I want to put the last two keys of an ordered dict d at 
the beginning, I would do it as d = d[:-2] + d[-2:].

With the list attribute (called "sequence" in odict, you would have to 
write: d.sequence = d.sequence[:-2] + d.sequence[-2:]. This is not only 
longer to write down, but you also have to know that the name of the 
attribute is "sequence". Python's strength is that you don't have to 
keep many details in mind because it has a small "basic vocabulary" and 
orthogonal use. I prefer the ordered dictionary does not introduce new 
concepts or attributes if everything can be done intuitively with the 
existing Python methods and operators.

-- Christoph



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