[Tutor] classes for setting 2D envelopes

[Matt Gregory]

Hi all,

I've been working on creating 2D bounding box (envelope) classes to 
describe spatial data.  Variations of these are available in other 
spatial libraries (e.g. Shapely), although I haven't found envelopes 
specific to raster data that also specifies cell size. Could be I just 
haven't found them yet.

I have two classes (Envelope and RasterEnvelope).  Envelope specifies a 
bounding box and does error checking (through __setattr__) when a 
coordinate is changed.  RasterEnvelope additionally specifies a cell 
size (along with n_rows and n_cols), does the bounds checking on a 
coordinate change and adjusts the spatial envelope accordingly (again 
through __setattr__).  I've posted the code (and some unit tests) here:

   http://pastebin.com/Twf3RjWa

So far things work, but I have a nagging feeling that there's too much 
work devoted to changing coordinates.  In another thread I posted a 
while back 
(http://mail.python.org/pipermail/tutor/2010-August/077940.html), Steven 
D'Aprano recommended using immutable types for point coordinate data and 
I'm guessing the advice might be applicable here as well?  If so, I'm a 
slow learner ;)  There are also (at least) a couple of design flaws 
right now that could be remedied if I put more work into it:

- A RasterEnvelope is 'pinned' by its upper-left coordinate and only 
changes to x_min or y_max will cause changes in this coordinate (by 
design but probably a limitation)

- Changing cell size currently doesn't change the corresponding window 
because there is no specification as to whether n_cols/n_rows should 
change or x_max/y_min should change.

Both these probably suggest creating new RasterEnvelope instances any 
time a coordinate changes?  Any feedback would be welcome, so that I 
don't devote too much more time down a bad route.

thanks, matt