python中contextmanager修饰的方法

        from contextlib import contextmanager
 @contextmanager
def demo():    
print '[Allocate resources]'    
print 'Code before yield-statement executes in __enter__'    
yield '*** contextmanager demo ***'    
print 'Code after yield-statement executes in __exit__'    
print '[Free resources]'
 
with demo() as value:    
print 'Assigned Value: %s' % value