strange side effect with lists!?

[Wolfgang.Stoecher at profactor.at]

Hello,

I'm new to Python and playing around. I'm confused by the following 
behaviour:

>>> l1 = [1] # define list
>>> l2 = l1  # copy of l1 ?
>>> l2,l1
([1], [1])
>>> l2.extend(l1) # only l2 should be altered !?
>>> l2,l1
([1, 1], [1, 1]) # but also l1 is altered!

So what is the policy of assignment? When is a copy of an object created? 
Where to find dox on this?

thanx,

Wolfgang