How to organize source code and "import"s???

[Tian]

I am writing a python program which needs to support some plug-ins. I
have an XML file storing some dynamic structures. XML file records some
class names whose instance needs to be created in the run time while
parsing the XML file. I wonder what is the best solution for this
problem?

I also have some problem about the "import". How should I design my
packages?
Say, I have all code locates at c:\projects\sami, "c:\project" is in my
PYTHONPATH environment variable. Suppose my folder structure is like
this:

c:
projects\             <-----this directory is in PYTHONPATH
          sami\
                 __init__.py
                 main.py
                 xmlparser.py
                 window.py
                 proc.py
                 support\
                          __init__.py
                          helper.py
                 plugins\
                          __init__.py
                          BaseClass.py    <---no instance for this one
                          ExtClassA.py
                          ExtClassB.py
                          ExtClassC.py
                          ExtClassD.py



Each file in \projects\sami\plugins contains a class with a same name
as the file, (ExtClassA.py has class ExtClassA), the instance of these
classes need to be created at runtime while functions in xmlparser.py
is parsing an XML file. main.py is the start point of the program.

Other files in the \projects\sami\ and projects\sami\support are
supporting modules.

1. What should I write in each __init__.py ???
   In "main.py", if I need functions in "proc.py", should I write
"import proc"?  If I need functions in "helper.py", can i write "import
support.helper"?? what else should I do to support all these?

2. What is the best way to make instance of a class from a string type
name? One method I have successfully tried is using "from SomeWhere
import *", then get class from globals() and make instance. But How can
I import all "ExtClass?.py"? Where should I write these import codes?