Need help with Python scoping rules

[Jean-Michel Pichavant]

kj wrote:
>
> I have many years of programming experience, and a few languages,
> under my belt, but still Python scoping rules remain mysterious to
> me.  (In fact, Python's scoping behavior is the main reason I gave
> up several earlier attempts to learn Python.)
>
> Here's a toy example illustrating what I mean.  It's a simplification
> of a real-life coding situation, in which I need to initialize a
> "private" class variable by using a recursive helper function.
>
> class Demo(object):
>     def fact(n):
>         if n < 2:
>             return 1
>         else:
>             return n * fact(n - 1)
>
>     _classvar = fact(5)
>
> This code fails to compile, with the error "global name 'fact' not
> defined".
>   
[snip]

fact is defined within the Demo class, so to access it, you'll need to 
prefix it with Demo:

_classvar = Demo.fact(5).

The problem is, Demo will raise a NameError exception.
The solution is pretty simple, cause your fact function seems to be 
unrelated to the Demo class :

def _fact(n):
    # some code

class Demo(object):
    _classvar = _fact(5)


It may happen that you *want* your fact within the Demo, in you example 
it's not that obvious, but I think Java would allow such following patern:

class Color:
    def __init__(self, r, g,b):
          pass
    BLACK = Color(0,0,0)

It make sens from a design point of view to put BLACK in the Color 
namespace. But I don't think it's possible with python.
You could trick the system using inheritance:

class Chrome:
    def __init__(self, r,g,b)
       pass
Putting all your code in the Chrome class then:

class Color(Chrome):
    BLACK = Chrome(0,0,0)

I'm not sure this is satisfying.

JM