Dear All,

I have just read PEP-320 and noticed the following comments regarding

the "collections" package:

...

- ? bag (only if use cases established)

...

I would like to argue for such a use case. We at designtheory.org are

working on a (maths / stats / comp. sci.) project in the area of Design

Theory. Design theory intersects with many fields, to mention a few:

combinatorics, graph theory, finite geometry, coding theory, and the

design of statistical experiments (from which then name of the field

comes). Most of our software development will be in Python, although

the released python software so far is very modest.

To show why bags are important for us and, in general, for discrete

mathematicians, here is the definition of the most important type of

designs. A binary block design is a multiset (that is a 'bag') of

subsets of a 'base' set; the elements of the base set are called

'points' and its subsets are called 'blocks'. (Personally, I prefer the

name bag but most mathematicians use multiset.) A non-binary block

design is one whose blocks can also be multisets.

The computations we are facing are very much combinatorial in nature so

we are happy to see that Sets became a builtin and, of course, would

like to see a C implementation for bags too. Our pydesign package (will)

heavily use C extensions. So far we use numarray to compute statistical

properties of designs, but many more combinatorial functionalities will

be implemented in C.

In particular, we are planning to implement a basic permutation group

package whose core will eventually be a C extension. We would like to

deal with automorphism groups and isomorphisms of designs on at least a

basic level within the pydesign package without having to resort to

specialized mathematical packages like GAP. These functionalities can be

useful not only for design theorist, but for anybody using Python to

deal with combinatorial structures, like graphs for example.

In all of these areas the use of multisets (bags) is pervasive.

Please, implement it -- so we don't have to :-)

For more details on our project, please visit

http://designtheory.org/
-- ,

Peter Dobcsanyi