old_contents: 
  def occurrences(s, t):
      if len(t) > len(s):
          return None
  
      results = []
  
      for i, _ in enumerate(s):
          if s[i:i + len(t)] == t:
              results.append(i + 1)
  
      return results
  
  
  if __name__ == '__main__':
  
      with open('data/rosalind_subs.txt') as f:
          s = f.readline().rstrip()
          t = f.readline().rstrip()
  
      print ' '.join("%s" % s for s in occurrences(s, t))
new_contents: """
  Given: Two DNA strings s and t (each of length at most 1 kbp).
  
  Return: All locations of t as a substring of s.
  """
  
  
  def occurrences(s, t):
      if len(t) > len(s):
          return None
  
      results = []
  
      for i, _ in enumerate(s):
          if s[i:i + len(t)] == t:
              results.append(i + 1)
  
      return results
  
  
  if __name__ == '__main__':
  
      with open('data/rosalind_subs.txt') as f:
          s = f.readline().rstrip()
          t = f.readline().rstrip()
  
      print ' '.join("%s" % s for s in occurrences(s, t))
subject: Add description to problem_subs.py