piedra papel o tijera con listas

def ppt(a, b):
  if a == "piedra":
    if b == "tijeras":
      return "gana a"
    elif b == "papel":
      return "pierde a"
    else:
      return "empate"
  if a == "papel":
    if b == "tijeras":
      return "gana b"
    elif b == "piedra":
      return "gana a"
    else:
      return "empate"
  if a == "tijeras":
    if b == "papel":
      return "gana a"
    elif b == "piedra":
      return "gana b"
    else:
      return "empate"
    
ppt("piedra", "tijeras")