It's notable that in the same situation perl will do basically the same thing as python ... unless it detects a dependency (i.e. same variable referenced on both sides), in which case it constructs a temporary list, fills that from the RHS, then evaluates the LHS to get a list of lvalues, then assigns (roughly, I've not been in that part of the VM code for a bit).

i.e. from perl's POV, python's behaviour is an optimisation, which if over-applied introduces a bug ... but, y'know, DWIM vs. regularity etc., I'm just surprised that python's behaviour is sloppier than perl's here :)