The only difference between your two solutions is the first one returns a list and the second, a tuple. Neither one is a generator.

Edit: That was wrong. TIL if you write what looks like a tuple comprehension, it's actually a generator.

To answer the original question though, if you write it as a generator with the yield keyword

  def square(nums: list):
    for num in nums:
      yield num * num

will consume less memory because it won't have the return value in memory (the list). You can verify this by putting a print statement after the yield - it won't be executed until you call next on the generator.