python bijection

[Carl Banks]

On Dec 4, 4:42 pm, Lie Ryan <lie.1... at gmail.com> wrote:
> On 12/5/2009 9:41 AM, Carl Banks wrote:
>
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> > On Dec 4, 12:46 pm, geremy condra<debat... at gmail.com>  wrote:
> > more common than full-blown graph package).
> >> Sure, its a tree, which is also a graph. In this case it looks to
> >> me more like a directed acyclic graph than anything, but its
> >> pretty much just semantics since the interface is functionally
> >> equivalent.
>
> > I'd have to agree with Lie, yes a tree is a graph, but it's simply not
> > an argument that Python community is grasping for graph structures.
> > It's like arguing that the Python community could benefit from a
> > quaternion type, because quaternions are actually heavily used in
> > Python, because a scalar number is a quarternion.
>
> > Carl Banks
>
> > (Would be +1 on a good graph implementation... just not because of
> > ElementTree.)
>
> I think this could be an interpretation of the Zen:
>
> Simple is better than complex.
> Complex is better than complicated.
>
> can be read as:
> List is better than Tree
> Tree is better than Graph
>
> not having Tree and Graph package in the standard library force most
> people to find List-based solution. And people that know they need
> graphs will find them in 3rd party modules. I have needed Trees a few
> times in python, but very rarely a Graph (except for playing around).
> YMDWV (your mileage definitely will vary).

If you want a better example, consider various database schemas that
have one-to-one, one-to-many, many-to-one, and many-to-many
relationships.  I daresay these are very common.  All of these can be
represented by a non-directed graph.

Another common use of directed graphs is for dependency
relationships.  In practice, a lot of times running things in order of
dependency is done by assigning everything a scalar priotity, and
executing in order of priority.  This can work ok, but it's fragile.
If there's a graph type in Python maybe people will be encouraged to
handle dependencies explicitly.


Carl Banks