from machine import Pin

keys = (("1", "2", "3", "A"),
       ("4", "5", "6", "B"),
       ("7", "8", "9", "C"),
       ("*", "0", "#", "D"))

rows = [18, 19, 20, 21]
swcols = [12, 13, 14, 15]
ledcols = [0, 1, 2, 3]

row_pins = [ Pin(pin_name, mode=Pin.OUT) for pin_name in rows ]
swcol_pins = [ Pin(pin_name, mode=Pin.IN, pull=Pin.PULL_UP) for pin_name in swcols ]

last_keys = set()

while True:
  current_keys = set()
  for current_row in range(len(row_pins)):
    for row_number, row in enumerate(row_pins):
      if current_row == row_number:
        row.low()
      else:
        row.high()
    for col_number, col in enumerate(swcol_pins):
      if col.value() == 0:
        current_keys.add(keys[current_row][col_number])
  if len(current_keys - last_keys):
    print("Pressed keys = ", current_keys - last_keys)
  if len(last_keys - current_keys):
    print("Released keys = ", last_keys - current_keys)
  last_keys = current_keys