The first is a transform of the values (with only that, the list stays the same length, the IF says whether the transform happens or doesn't for each value), th

The first is a transform of the values (with only that, the list stays the same length, the IF says whether the transform happens or doesn't for each value), the second is a WHERE filter picking or dropping some values (list might shrink), you can have both (list shrinks and the selected values are transformed): >>> list([ x if x > 1 else x*2 for x in a if x <3] ) [2, 4]