(Don\'t worry, this isn\'t another question about unpacking tuples.)
In python, a statement like foo = bar = baz = 5 assigns the variables foo, bar, and
bar = 5 is not an expression. The multiple assignment is a separate statement from the assignment statement; the expression is everything to the right of the right-most =.
A good way to think about it is that the right-most = is the major separator; everything to the right of it happens from left to right, and everything to the left of it happens from left to right as well.