old_contents: import app.change_mac_address as c
  
  
  def test_dec_to_hex():
      assert c.dec_to_hex(0) == '0'
      assert c.dec_to_hex(1) == '1'
      assert c.dec_to_hex(3) == '3'
      assert c.dec_to_hex(10) == 'a'
      assert c.dec_to_hex(11) == 'b'
      assert c.dec_to_hex(12) == 'c'
      assert c.dec_to_hex(15) == 'f'
  
  
new_contents: import re
  import app.change_mac_address as c
  
  
  def test_dec_to_hex():
      assert c.dec_to_hex(0) == '0'
      assert c.dec_to_hex(1) == '1'
      assert c.dec_to_hex(3) == '3'
      assert c.dec_to_hex(10) == 'a'
      assert c.dec_to_hex(11) == 'b'
      assert c.dec_to_hex(12) == 'c'
      assert c.dec_to_hex(15) == 'f'
  
  def test_get_hex_pair():
      """Checks that it only returns valid hex digits."""
      pattern = re.compile('([a-f]|\d)')
      hex_pair = c.get_hex_pair()
  
      assert len(hex_pair) == 2
      for hex_digit in hex_pair:
          assert pattern.match(hex_digit) is not None
  
  
  
subject: Add tests