nested classes

[gbreed at cix.compulink.co.uk]

In article <3B2E2D3A.75F329D at sympatico.ca>, seefeld at sympatico.ca 
(Stefan Seefeld) wrote:

> hi there,
> 
> I'd like to provide a hierarchy of config data by means of
> nested classes and static variables, such as:
> 
> class A:
>     prefix='usr/local'
>     class B:
>       datadir = A.prefix + '/share'
> ...
> 
> however, I get a NameError: name 'A' is not defined

See the earlier message "getting a reference to a class inside 
its definition" and the flood of no good ideas at all that 
followed it.

> What am I doing wrong ? The above is possibly a bit driven
> by my C++ programming style. What is the python way of doing
> this ?

You can get that to work in C++?  It looks strange to me, I'm 
sure you can't be trying to do what you think you're trying to 
do.  This is the nearest I can think of that works:

>>> class A:
...     prefix = 'usr/local'
...     
>>> class B(A):
...     datadir = A.prefix+'/share'
...     
>>> A.prefix
'usr/local'
>>> B.datadir
'usr/local/share'


You may also like to take a look at os.path.join

>>> import os.path
>>> class A:
...     prefix = os.path.join('usr','local')
... 
>>> class B(A):
...     datadir = os.path.join(A.prefix, 'share')
... 
>>> print A.prefix
usr\local
>>> print B.datadir
usr\local\share
>>> print B.prefix
usr\local

It'll give the right results on a UNIX system, and stop double 
or omitted slashes.

                       Graham