[Mapserver-dev] Issue with undefining string attributes using mapscript

[Sean Gillies]

Hello,

I just entered this as a bug in Bugzilla.

I'm going to use Python syntax in the description.  I'm convinced, but 
can't
prove, that this is an issue for the other flavors of SWIGged mapscript.

Say we have a mapfile that contains a layer (index = 0) and that no 
group
is defined for the layer.

We then create a mapscript mapObj using this mapfile.  When we try to
access the 'group' attribute of the layer like

    layer = mapobj.getLayer(0)
    group = layer.group

the variable 'group' has a value of None (or your own flavor of NULL).
This is exactly right.

Now, during use of the mapObj we might change the group attribute like

    layer.group = 'foo'

and this works fine.  However, we cannot now undefine the group 
attribute.
The underlying SWIG accessor functions will not allow us to do this

    layer.group = None

(which would be the most Pythonesque way to undefine an attribute) 
because
the layerObj_group_set function requires a string argument.  Setting the
group attribute to an empty string like

    layer.group = ''

does not undefine the attribute.

It's important to be able to undefine attributes because there are many
functions in the mapserver that execute differently depending on whether
or not string attributes evaluate as equal to NULL.

I figure that there are two possible solutions:

1) Get the SWIG accessor functions to use typemaps that map our 
scripting
languages None/NULL values to C NULLs.  Unfortunately, I haven't been 
able
to turn up any documentation or suggestions on how this would be done.
I know how to use typemaps when wrapping existing code, but suspect that
it's going to be different with the accessor functions that are 
generated
by SWIG.

2) Refactor the mapserver code so that empty strings are treated as 
NULL.

Are there better solutions?

--
Sean Gillies
sgillies at frii.com
http://www.frii.com/~sgillies