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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.