Creating methods depending on packages that have been installed

[C Gillespie]

Dear All,

What is the best way of creating methods depending on the packages that have
been installed.

A silly example is

try:
import somepack
class hello:
def __init__(self):
print 'hi'
def some(self):
print 'some'
except:
class hello:
def __init__(self):
print 'hi'

This is obviously not what I want - since it looks ugly and I wish to test
for lots packages.

Any ideas?

Thanks

Colin