关于什么：
import contextlib
def unzip(source, target):
with contextlib.closing(zipfile.ZipFile(source , "r")) as z:
z.extractall(target)
print "Extracted : " + source + " to: " + target
contextlib.closing确实执行了该__exit__方法ZipFile中应该缺少的方法。即调用close方法
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