Help with sets

[Terry Reedy]

On 10/5/2010 3:39 AM, Steven D'Aprano wrote:

> Sets aren't an alternative to dictionaries. They have a completely
> different purpose.

A dict/mapping is a specialized set -- a set of ordered pairs in which 
each first member (the 'key') only appears once as a first member. The 
set union of two mappings may not be a mapping, which is why dicts have 
a specialized .update method instead of .union.

> It so happens that sets and dicts *in Python* happen to share some
> implementation details, and they happen to share similar syntax, but
> don't be fooled by these incidental details. They could be implemented
> differently.
>
> Dicts are used when you need a 1:1 mapping between a key and a value:

The mapping does not have to be 1:1, which means that each value also 
appears only once, so that the mapping can be inverted. Values can 
appear more than once, which is to say, many keys can map to one value. 
so the mapping is many:1. Collections (sets or lists) are used as values 
to implement 1:many mappings.

-- 
Terry Jan Reedy