L1 = ['Hello', 'World', 18, 'Apple', None]

# L2 = [s.lower() for s in L1 if isinstance(s,str) ==True]

L2=[]

for x in L1:

  if isinstance(x,str)==True:

     L2.append(x.lower())

print(L2)

if L2 == ['hello', 'world', 'apple']:

    print('测试通过!')

else:

    print('测试失败!')