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My understanding of the augmented assignment 
statements is that they should always assign to the 
variable that they read from. However, consider the 
following Python session:
 >>> class A: x = 1 
 ... 
 >>> a=A()
 >>> a.x += 1
 >>> a.x, A.x
 (2, 1)
Here, the expression "a.x += 1" read from a class 
variable, and wrote to an instance variable. A 
similar effect can occur within a member function:
 >>> class A:
 ... x = 1
 ... def f(s): s.x += 1
 ... 
 >>> a = A()
 >>> a.f()
 >>> a.x, A.x
 (2, 1)
I have elicited this behavior in Python 2.0 and 2.1 
under Linux (Redhat 6.0), and Python 2.0 on sunos5.