python中os.listdir的排序问题

os.listdir(os.getcwd())

>>> files=os.listdir(".")
>>> files.sort()
>>> print files
['1.txt', '10.txt', '11.txt', '2.txt', '3.txt']
>>> files.sort(key= lambda x:int(x[:-4]))
>>> print files
['1.txt', '2.txt', '3.txt', '10.txt', '11.txt']