> In fact, the language designers even took care (for the sake of comprehensibility :) to ensure that the semantics of the two fors (in a two-level nested listcomp) have the same meaning (aka order of priority) as a two-level nested for loop, i.e. outer for wraps inner for, in both cases. It's mentioned in the spec somewhere.

Yeah, I think that was a huge mistake, because the most common case is one-level and if you have one level the syntax is just the reverse of a regular for, but in a two-level comprehension (the rare case) you have to remember that it's not a reversal, it's a cyclic permutation.

i.e. most of the examples you see look like this, and these two are equivalent:

    [y for i in x]
    for i in x: y

and then occasionally you see a comprehension that looks like:

    [z for j in y for i in x]

so you'd expect this to be equivalent to

    for i in x: for j in y: z

but it isn't, it's equivalent to

    for j in y: for i in x: z

which is really hard to remember.