python如何关闭已经gc'ed的文件？

with open('file.txt', 'r') as f:
    # do something with the file
    # the file will be automatically closed when the block exits

# or

f = open('file.txt', 'r')
try:
    # do something with the file
finally:
    f.close()
    # the file will be closed no matter what happens in the try block