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Sorry, but your understanding is flawed.  Changed the 
category to Documentation and assigned to Fred, because I 
agree the Ref Man isn't clear enough here:  "The target is 
evaluated only once" is certainly how Guido *thinks* of it, 
but it really needs a by-cases explanation:

For an attributeref target, the primary expression in the 
reference is evaluated only once, but a getattr is done on 
the RHS and a setattr on the LHS.  That is,

    e1.attr op= e2

acts like

    temp1 = e1
    temp2 = e2
    temp1.attr = temp1.attr op temp2

Etc.  It's going to take some real work to write this up so 
they're comprehensible!  I recommend skipping most words 
and presenting "workalike" pseudo-code sequences instead.  
That will take some sessions with the disassembler to be 
sure the pseudo-code is 100% accurate.