A scoping question

[Steven Bethard]

It's me wrote:
> This must be another newbie gotchas.
> 
> Consider the following silly code, let say I have the following in file1.py:
> 
> #=============
> import file2
> global myBaseClass
> myBaseClass = file2.BaseClass()
> myBaseClass.AddChild(file2.NextClass())
> #=============
> 
> and in file2.py, I have:
> 
> #=============
> global myBaseClass
> class BaseClass:
>     def __init__(self):
>         self.MyChilds = []
>      ...
>     def AddChild(NewChild):
>         self.MyChilds.append(NewChild)
>     ...
> class NextClass:
>     def __init__(self):
>         for eachChild in myBaseClass.MyChilds:  # <- ERROR
>             ...
> #=============
> 
> When I run this, Python complains that myBaseClass is undefined in the last
> line above.
> 
> What am I doing wrong?  (Yes, I know I am thinking too much in C).  I
> thought the global declaration would have been sufficient but it's obviously
> not.

I think you're confused about what the global keword does.  Declaring a 
name as global makes that name global *to the module*:

http://docs.python.org/ref/global.html
http://docs.python.org/lib/built-in-funcs.html#l2h-32

What you probably want instead is:

-------------------- file1.py --------------------
import file2
myBaseClass = file2.BaseClass()
myBaseClass.AddChild(file2.NextClass())
--------------------------------------------------

-------------------- file2.py --------------------
class BaseClass:
     def __init__(self):
         self.MyChilds = []
     def AddChild(self, NewChild):
         self.MyChilds.append(NewChild)
class NextClass:
     def __init__(self):
         from file1 import myBaseClass  # IMPORT
         for eachChild in myBaseClass.MyChilds:
             pass
--------------------------------------------------

Note that I import myBaseClass in __init__.  If I imported it at the top 
of the module, then file1 would import file2 which would then import 
file1 and you'd have a circular dependency.

As it is, your code is very tightly coupled.  Why don't you put all this 
code into a single module?

Steve