I think I was personally surprised by two assumptions that I implicitly held, both of which disappear when you write them down: 1. The syntax of comprehensions

I think I was personally surprised by two assumptions that I implicitly held, both of which disappear when you write them down: 1. The syntax of comprehensions led me to think that they introduce some kind of a variable binding, such as `let` in many Lisps, and that binding `lookup[x]` would have been a syntax error. Of course Python didn't introduce anything like that for comprehensions, and it's just a normal assignment operation, with an extra scope inserted in Python 3 to stop `x` leaking into the enclosing scope. 2. Because of the swap idiom `x, y = y, x` I thought tuple assignment happens somehow in parallel, so my code should have caused an undefined variable error, even if it is not a syntax error. But of course it just evaluates the right hand side first and then assigns to the left hand side in left-to-right order, which the language reference https://docs.python.org/3/reference/simple_stmts.html#assignment-statements https://docs.python.org/3/reference/simple_stmts.html#assign... notes "sometimes result[s] in confusion" (near the end of the section).