Not the same kind of nesting, is it?

  [[None for _y in range(y)] for _x in range(x)]

for two dimensions is fine, while

  [None for _x in range(x) for _y in range(y)]

(to create a flattened 2-dim list? excuse the dummy example) is just begging for later headache. Explicit looping is much more readable. Note the flipped order of expressions -- these two statements create the same iterators.

Not to mention, you can even reuse the name for extra evil:

  [i for i in range(2) for i in range(3)]

results in

  [0, 1, 2, 0, 1, 2]