Why is dictionary.keys() a list and not a set?

[Christoph Zwerschke]

Mike Meyer wrote:
> Are you sure dict.values() should be a set? After all, values aren't
> guaranteed to be unique, so dict(a = 1, b = 1).values() currently
> returns [1, 1], but would return set([1]) under your proposal.

Good point. Contrary to the keys, the values are not unique. Still, it 
would make sense to only return the set (the value set of the mapping) 
because this is usually what you are interested in and you'd think the 
information about which value belongs to which key is lost anyway.

However (see other postings in this thread) this last statement is 
wrong: If you call keys() and values() consecutively, then they are 
guaranteed to correspond to each other. If values() would return a set, 
this would be completely broken.

> What about dict.items()?

Actually, if keys() would be a set, this should return a set, too (the 
set of key/value tuples).

>>For instance, by allowing the set operator "in" for dictionaries,
>>instead of "has_key".
> "in" already works for dicdtionaries:

I know. I wanted to use it as an example where set operations have 
already been made available for dictionaries.

I know, 'in' has existed for lists and tuples long before sets, but 
actually it is a native set operation.

 From a mathematical point of view, it would have been nice if Python 
had defined "set" as a basic data type in the beginning, with lists, 
tuples, dictionaries as derived data types.

By the way, i wonder why the common mathematical notation { 1,2,3 } was 
not allowed for set((1,2,3)). It would not clash with the dictionary 
notation which requires additional colons.

-- Christoph