input: Write a function to find the common elements between two given lists. Return the list of common elements. If there are no common elements, return an empty list.
  assert common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert common_elements([1, 2, 3], [4, 5, 6])==[]
  assert common_elements([], [1, 2, 3])==[]
  assert common_elements([1, 1, 1], [1, 1, 1])==[1, 1, 1]
  assert common_elements([], [])==[]
output: def common_elements(list1, list2):
      return [item for item in list1 if item in list2]