On Thu, Jul 15, 2010 at 8:26 PM, mike bayer <report@bugs.python.org> wrote:
..
> where is it defined that sets are not "supposed" to contain mutable items? such a requirement vastly limits the usefulness of sets.
>
Well, there is no such requirement. The actual requirement is that
they should be hashable. For built-in types, however hashable is the
same as immutable. Arguably, user-defined classes should emulate
that.
> Consider that relational database rows are mutable. A result set containing multiple rows which each have a primary key comprises a set, hashed on primary key. But other attributes of each row can be updated. Surely this is not controversial ? What Python data structure should I (and a whole bunch of Python ORMs) be using to represent mutable, relational database rows, unordered and unique on primary key, in memory ?
Why wouldn't you represent a result set as a dict mapping primary key
(tuple) to list of column values?