Programming

1. Stop using bullshit variable names. Is c common? Then call it common. Names matter.
   
2. It doesn't matter which list is longer or which list you iterate over. If you were trying to optimize, all you did was waste time and create a bug.
   
3. List comprehensions are always introduced as being the same thing as a list appending loop, but it isn't true. You can't reference the resulting list from within the comprehension so what you're trying to do will never work. To get unique items, either make the input or output unique.
   
4. Randomly generating lists and manually checking the result is a bad joke. Instead, organize code into small functions and write test cases:
   

def make_common_list(a, b):
  common = [i for i in a if i in b]
  return make_unique_list(common)

def make_unique_list(seq):
  unique_list = []
  for i in seq:
    if i not in unique_list:
      unique_list.append(i)
  return unique_list

def test_make_common_list():
  assert make_common_list([1,1,2], [1,2,5]) == [1, 2]
  assert make_common_list([1,2,5], [1,1,2]) == [1, 2]
  assert make_common_list([], [1,2,5]) == []

1. The list names are used from the exercise given. Also, I didn't think they would matter much because they aren't being used in any real world code. Just as an exercise to learn how things work. I get that if I were building something that would be used for anything other than completing this coding exercise naming the lists something useful would be important. I have a little tiny bit of knowledge of JavaScript and have dicked around with creating things in it and I know to always make sure an array has a useful name so that I know what data it's storing.