Thinking about it some more, the surprising behavior here is pretty much just that it does the assignments in left-to-right order instead of right-to-left order. If you try and break down `a = b = expr` into a series of binary assignment operators, the natural answer is `a = (b = expr)` (b doesn't exist yet so you can't write `(a = b) = expr`) where `(b = expr)` behaves as though it returns the value again. And I believe this is in fact how it works in Ruby (as Ruby assignments return the value). But Python assignments don't actually return the value, as they are statements and not expressions.

Presumably Python does the assignments in left-to-right order because any observable side-effects in the evaluations of a and b would be expected to be in left-to-right order. It just results in counterintuitive behavior like this.